Add default proguard flags from guava

Guava 27.1 causes some warnings in R8, silence them.

Bug: 130306229
Test: m checkbuild
Change-Id: Ia9948d597538a16c5b7ab26a2e41eebde52e7427
Merged-In: Ia9948d597538a16c5b7ab26a2e41eebde52e7427
Exempt-From-Owner-Approval: cherry pick
(cherry picked from commit b4917146)