Enable R8 by default (sixth attempt)

Fixed bugs which was discovered in the last attempt.

Bug: 72373167
Bug: 72391662

Test: m -j PRODUCT-taimen-userdebug
Test: Check that some .vdex files built have R8 generated code
Change-Id: I89aaed9a4156748a513d6ee0284068b8a3c52322