Add stl property to android_app.

The flag is a Soong version of LOCAL_NDK_STL_VARIANT in apps and app
tests. Unlike in the case of cc_library and its siblings, the only
meaningful value for the make var when it's used in apps is
'c++_shared', in which case we add additional dependency to libc++.

Fixes: 130891985
Test: app_test.go + CtsNdkBinderTestCases
Change-Id: I83f45d375742164fff7f160a734b0686e56b5c38