Assessment of adoption r8 to take place proguard in Android build.

[dapengzhang0]

[dapengzhang0]

As suggested by #5531 (comment)

[dapengzhang0]

cc @ejona86

[ejona86]

We know our users are still using proguard, and lots of failures have been proguard-specific. So it seems we need to retain using proguard. We could potentially use r8 as well, or as a separate build/test.