Don't build targets if they're not used.

Add functionality in build_test_suites.py to not build targets if
their outputs are not used in the test configurations saved in the build
context. If none of the tests reference the targets' outputs they will
not be built at all.

Note that the corresponding flags will need to be enabled for these
optimizations to take place.

Test: atest optimized_targets_test
Bug: 348489774
Change-Id: I8f0ac90e75552ae80073f13229b026c7f23476a6