Revert "Revert "Use D8 by default for android_test""

This reverts commit 4e445be5.

Reason for revert: GtsExoPlayerTestCases now uses multidex.

Test: m + manual builds for test_suites_x86_64_coverage
Change-Id: I608b3b79137b216f546446e1c4f89f4542572581
Bug: 192032291