This PR increases Android's `minSdkVersion` to 21.

There are two changes in this PR aside from simply increasing the number
from 19 to 21 everywhere.

First, tests using `flutter_gallery` fail without updating the
lockfiles. The changes in the PR are the results of running
`dev/tools/bin/generate_gradle_lockfiles.dart` on that app.

Second, from
[here](https://developer.android.com/build/multidex#mdex-pre-l):
> if your minSdkVersion is 21 or higher, multidex is enabled by default
and you don't need the multidex library.

As a result, the `multidex` option everywhere is obsolete. This PR
removes all logic and tests related to that option that I could find.
`Google testing` and `customer_tests` pass on this PR, so it seems like
this won't be too breaking if it is at all. If needed I'll give this
some time to bake in the framework before landing the flutter/engine
PRs.

Context: https://github.com/flutter/flutter/issues/138117,
https://github.com/flutter/flutter/issues/141277, b/319373605

Reland "Fix issue where DevTools would not be immediately available when using --start-paused (#126698)" (#129368)

**Original Description:**

> Service extensions are unable to handle requests when the isolate they
were registered on is paused. The DevTools launcher logic was waiting
for some service extension invocations to complete before advertising
the already active DevTools instance, but when --start-paused was
provided these requests would never complete, preventing users from
using DevTools to resume the paused isolate.
> 
> Fixes https://github.com/flutter/flutter/issues/126691

**Additional changes in this PR:**

The failures listed in https://github.com/flutter/flutter/pull/128117
appear to be related to a shutdown race. It's possible for the test to
complete while the tool is in the process of starting and advertising
DevTools, so we need to perform a check of `_shutdown` in
`FlutterResidentDevtoolsHandler` before advertising DevTools.

Before the original fix, this check was being performed immediately
after invoking the service extensions, which creates an asynchronous gap
in execution. With #126698, the callsite of the service extensions was
moved and the `_shutdown` check wasn't, allowing for the tool to attempt
to advertise DevTools after the DevTools server had been cleaned up.

---------
Co-authored-by: Zachary Anderson <zanderso@users.noreply.github.com>

Revert "Fix issue where DevTools would not be immediately available when using --start-paused" (#128117)

Reverts flutter/flutter#126698

There are a bunch of tool crashes on CI that start with this commit. I'm
not sure this PR is the cause because there is no backtrace from the
tool on the crashes. The only error message is `Oops; flutter has exited
unexpectedly: "Null check operator used on a null value`.

Service extensions are unable to handle requests when the isolate they were registered on is paused. The DevTools launcher logic was waiting for some service extension invocations to complete before advertising the already active DevTools instance, but when --start-paused was provided these requests would never complete, preventing users from using DevTools to resume the paused isolate.

Fixes https://github.com/flutter/flutter/issues/126691

Sync lints with https://github.com/dart-lang/linter/blob/master/example/all.yaml and enable `implicit_reopen` and `type_literal_in_constant_pattern` (which have no violations). Also contains some clean-up work towards enabling `matching_super_parameters`, which is not quite ready yet due to its handling of "private" arguments.

This reverts commit 275ab9c6.

Revert "Reland "Remove references to Observatory (#118577)""

This reverts commit 298d8c76.

This reverts commit 2df140f4.

Observatory is being deprecated for Dart 3.0 so it should no longer be
referenced in tooling messaging / flags.

See https://github.com/dart-lang/sdk/issues/50233

This reverts commit 254a796b.

Revert "Reland "Add --serve-observatory flag to run, attach, and test (#118402)" (#119529)" (#119729)

This reverts commit 7477d7ac.

* Reland "Add --serve-observatory flag to run, attach, and test (#118402)"

This reverts commit 86ab01d2.

* Fix flaky failures

* Fix VM service disappearing failure

This reverts commit 6cd4fa45.

This flag will allow for Observatory to be served by the VM service once it is disabled by default in the Dart SDK.

[flutter_tools] refactor devtools handler to expose single method for run/attach and restart (#75807)

* opt out the flutter tool

* oops EOF

* fix import

* Update tool_backend.dart

* Update daemon_client.dart

* fix more

* Add connectedVmServiceUri service extension and set from flutter_tools

This reverts commit 388dcd24.

This reverts commit 1cb0a24a.

* [flutter_tools] Serve DevTools at app start

Ensure attaching to an application with an existing DDS instance is not treated as a fatal error (#70847)