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
```

まず grpc-android、grpc-cronet、grpc-android-interop-testing に対して lint を実行し、次に android/build/reports 配下に生成されたレポートと、対応するモジュールのレポートを確認します。CheckResult、old-version、UI の警告を原因ごとに分類し、安全に抑制できるものを特定します。報告された問題のうち、実質的な範囲を文書化したうえで解決し、残りの作業をフォローアップ issue に分離できれば完了です。

索引モデルが issue の本文から書いたものです。