Fix lint issues in Android projects

It seems we aren't targeting the a recent Android version. Many of the CheckResult failures appear to be expected behavior, so it will need to be silenced in some way. There's some warnings about old versions and some UI goo. Quite a mixed bag. I'm fine if we resolve a substantial portion of them with this issue and open separate issues for the remaining items.

```
> Task :grpc-android:lint
Ran lint on variant debug: 1 issues found
Ran lint on variant release: 1 issues found
Wrote HTML report to grpc-java/android/build/reports/lint-results.html
Wrote XML report to grpc-java/android/build/reports/lint-results.xml

> Task :grpc-cronet:lint
Ran lint on variant debug: 1 issues found
Ran lint on variant release: 1 issues found
Wrote HTML report to grpc-java/cronet/build/reports/lint-results.html
Wrote XML report to grpc-java/cronet/build/reports/lint-results.xml

> Task :grpc-android-interop-testing:lint
Ran lint on variant release: 64 issues found
Ran lint on variant debug: 64 issues found
Wrote HTML report to grpc-java/android-interop-testing/build/reports/lint-results.html
Wrote XML report to grpc-java/android-interop-testing/build/reports/lint-results.xml
```

Start by running lint for grpc-android, grpc-cronet, and grpc-android-interop-testing, then inspect the generated reports under android/build/reports and the corresponding module reports. Group the CheckResult, old-version, and UI warnings by cause and identify which can be silenced safely. Done means resolving a substantial, documented portion of the reported issues and separating the remaining work into follow-up issues.

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