Added support for using R8 instead of Proguard + dx

With this change building with USE_R8=true will run R8 instead of Proguard + dx
for all targets that have LOCAL_PROGUARD_ENABLED set. R8 (through the
compatproguard wrapper) will use the same options as Proguard for now.

Additional Proguard configurations for R8 can be added using LOCAL_R8_FLAG_FILES
on individual targets. This makes it possible to make configuration updates
without touching the Proguard + dx configuration

Test: m -j
Test: m -j USE_D8=true USE_R8=true
Test: m -j USE_D8=true USE_R8=false
Test: m -j USE_D8=true
Change-Id: I78e8a0fd17cc5cefedccffe95b93c94a60a727f1