This removes direct file access from within flutter_tools
in favor of using `package:file` via a `FileSystem` that's
accessed via the `ApplicationContext`.

This lays the groundwork for us to be able to easily swap
out the underlying file system when running Flutter tools,
which will be used to provide a record/replay file system,
analogous to what we have for process invocations.

With this change, they're run via instance methods on an object
obtained through the context. This will allow us to substitute
that object in tests with replay/record versions to allow us to
mock out the os-layer in tests.

Artifacts from local engine builds will be found based on the --local-engine flag

Update flutter_tools for the "unoptimized" engine build flag and the new output directory naming scheme (#3832)

* rename the --develop option to --debug

* fail if both --debug and --deploy are specified

* download android-arm-deploy

* wire up --deploy to android-arm-deploy

* fix interpolation

* use the new cache code to locate more artifacts

* add todo

* review comments