Use a minimal set of mutators, module types, and singletons for tests

Calling android.NewContext() in tests results in a context that
contains all the mutators, module types, and singletons, which
causes unexpected interactions in unit tests.  Create an empty
context instead, and add in only the necessary mutators, module
types, and singletons.

Bug: 36366816
Test: soong tests
Change-Id: Ic61262c37e3436b3ad4ccaca18b737021c304be6