Enable R8 by default (second attempt for Q)

Fixed bugs which was discovered in the last attempt.

Bug: 74508478

Test: m -j PRODUCT-taimen-userdebug
Test: m -j PRODUCT-elfin-userdebug
Test: Check that some .vdex files built have R8 generated code

(cherry picked from commit a3b796d9)

Change-Id: Ibe08137cd6db94b1fc487267e169d3f5ee598846
Merged-In: I2a8e0e5cad1d698ae73ba152e34a08a496112d7d