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
diff --git a/ci/Android.bp b/ci/Android.bp
index 104f517..22c4851 100644
--- a/ci/Android.bp
+++ b/ci/Android.bp
@@ -76,6 +76,7 @@
     srcs: [
         "build_test_suites.py",
         "optimized_targets.py",
+        "build_context.py",
     ],
 }