Refactor OptimizedBuildTarget and add general-tests optimization

Refactor the OptimizedBuildTarget base class to more simply handle
enabling/disabling build optimizations based on flags. Now all
optimizers (aside from the no-op NullOptimizer will have their enabling
built into the base class, and also necessitate their get_build_targets
and package_outputs functions to be defined in order to not error out.
Add the GeneralTestsOptimizer class as well, unimplemented for now.

Test: atest build_test_suites_test
Bug: 358215235
Change-Id: Ide22c64fc238d754db8d8e76484947401e54e680