Allow selective R8 optimization for eng test_suites

Eng builds implicitly run R8 in debug mode. This is typically fine, but
test_suites can be built in eng mode, and some tests exercise behavior
that may require R8 optimizations. For example, annotation tests that
check the effective dex output in the presence of such annotations.

Allow a target to override this default behavior by adding "--release"
to its dxflags property.

Bug: 222468116
Test: atest InternalAnnotationsTests
Change-Id: Ie3328f1b56a6fe7c9f331281e6527e40f17f9271