[various] Scrubs pre-SDK-21 Android code (#9112)
- Removes any annotations or runtime checks related to Android API 20 or 21, as 21 is the minimum API version supported by the versions of Flutter these plugins support.
- Removes comment/README references to limitations for <21.
- Updates READMEs for app-facing packages to reflect the current support levels.
Fixes https://github.com/flutter/flutter/issues/157106
## Pre-Review Checklist
[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.
[various] Enable warnings as errors, and all warnings, for Android lint (#3648)
For all Android plugins:
- Enable all warnings for lint
- Treat all warnings as errors for lint
This significantly increases the scope of issues that we'll catch in CI. To
allow enabling this without having to make tons of fixes first, so that
we get the incremental benefit immediately, this adds new baselines for
all plugins. We can incrementally clean those baselines up over time.
(In practice we haven't prioritized that, but it would be good to start
paying down that technical debt incrementally at some point.)
See https://github.com/flutter/flutter/issues/88011