Simplify how SetFlagsRule provides defaults to FakeFeatureFlagsImpl

NOTE: This also updates the CustomFeatureFlags and FakeFeatureFlagsImpl inside the test directory to (almost) exactly match the current code generation. The only difference is that the "isFlagReadOnlyOptimized" returns true for the read only flags even without optimization enabled.

Fixes: 337449122
Flag: test_only
Test: atest FlagJUnitHostTests
Change-Id: Ib35db6a1c51fd5702ad6536a65c1e40e7b45d393
7 files changed