Use D8 by default for android_test

android_test defaults to using R8, but with shrinking, optimization and
obfuscation disabled, eliminating most of the benefits of R8. Instead,
use D8 by default, improving build performance and avoiding any other
issues that may arise in test-specific code related to whole-program R8
execution. An initial audit shows that android_test targets that *do*
enable shrinking or optimization also explicitly opt in to R8.

A follow-up CL will do the same for android_test_helper_app, but that
requires some additional auditing of downstream targets.

Bug: 192032291
Test: m + presubmit
Change-Id: I5b14a0986dde210f241a77c3a93daacf9e53d667