Cleanup a.t.TestGrouping

TestGrouping is not part of the API but is used by classes that are and
so will need to be included in the static library we ship as part of the
SDK. As that library will be built directly from the source (as opposed
to android.jar which is built from stubs) developers will be able to see
classes and methods that are hidden and so not present in the stubs.

This change makes TestGrouping and its members as inaccessible as
possible. Unused methods and fields were removed and the TestGrouping
class was simplified by passing the ClassLoader into the constructor,
initializing the classLoader, making the classLoader field final,
and removing the setClassLoader(ClassLoader) method.

Bug: 30188076
Test: make checkbuild
Change-Id: Ie0545b1a61b74cdbc78667417f360b5197e2e128