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
```

Comienza ejecutando lint para grpc-android, grpc-cronet y grpc-android-interop-testing; después, inspecciona los informes generados en android/build/reports y los informes de los módulos correspondientes. Agrupa las advertencias de CheckResult, old-version y UI por causa e identifica cuáles se pueden silenciar de forma segura. Se considera terminado cuando se haya resuelto una parte sustancial y documentada de los problemas reportados y el trabajo restante se haya separado en issues de seguimiento.

Escrito por el modelo de indexación a partir del texto del issue.