add BuildContext class and fix enabled features

Add a BuildContext class to simplify parsing the build context dict, and
fix the parsing of enabledBuildFeatures, which was being parsed as a
list of strings when it's actually a list of dicts like:
[{'name': '<feature_name>'}]

Test: atest build_test_suites_test
Bug: 361605425
Change-Id: I6424c444daf1582e92313c39f43207cb274aa78f