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As a result, the `multidex` option everywhere is obsolete.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