[build] rename android platforms to match CPU arch

[jdcormie]

Work around for bazelbuild/bazel#15837 (comment)

Fixes a problem where the example client and server APKs would build but could not be installed (INSTALL_FAILED_NO_MATCHING_ABIS: Failed to extract native libraries)

[sifmelcara]

Thanks!

1. I think we should move these platform declaration to subfolder if we want to remove android_ prefix, maybe bazel/platforms/android/BUILD. (In google's internal repo we declare them at //.../platforms/android/BUILD)
2. Change PR and commit message title to [build] Rename android platforms to match CPU arch

[jdcormie]

Thanks!

1. I think we should move these platform declaration to subfolder if we want to remove android_ prefix, maybe bazel/platforms/android/BUILD. (In google's internal repo we declare them at //.../platforms/android/BUILD)
2. Change PR and commit message title to [build] Rename android platforms to match CPU arch

done

[sifmelcara]

Some tests are still failing after rerun, but the failure doesn't look like relating to this change