Enable R8 by default (fourth attempt)

Fixed bugs which was discovered from the last attempt.

Bug: 71629503
Bug: 69825683
Bug: 71630128
Bug: 69445518
Bug: 71604169
Bug: 70886092

Test: m -j PRODUCT-taimen-userdebug
Test: Check that some .vdex files built have R8 generated code
Change-Id: I8b468dee5a5d63f1caa2c9a8b7b0ce5655e512b9