Reland "Android: Turning on R8 instead of Proguard for public targets"

This is a reland of 09f15bdb07cd457c1a96b08ce54faa4ba2771dee

Original change's description:
> Android: Turning on R8 instead of Proguard for public targets
>
> We are in the process of migrating to R8 to replace Proguard. This is
> the first step - move all usages of Proguard in public targets to R8.
>
> Some refactorings were natural with the few forced changes this caused.
>
> TBR=smaier
>
> Bug: 908988, 913554
> Change-Id: I2139919598fba1643d7560dc5557d5efb9a5887c
> Reviewed-on: https://chromium-review.googlesource.com/c/1357306
> Reviewed-by: Sam Maier <smaier@chromium.org>
> Reviewed-by: agrieve <agrieve@chromium.org>
> Commit-Queue: Sam Maier <smaier@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#619471}

TBR=proguard.flags(smaier)

Bug: 908988, 913554
Change-Id: I93690ad2a6838025221b7fab4b0bee1b06c920c9
Reviewed-on: https://chromium-review.googlesource.com/c/1394387
Commit-Queue: Sam Maier <smaier@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619702}
7 files changed