Android: cleaning up some Proguard flags

Making these keep rules more explicit.

Bug: 887942
Change-Id: I70038c8c58025892a7003fad5bb3ff83384f13f5
Reviewed-on: https://chromium-review.googlesource.com/c/1366262
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614508}
1 file changed