android.test.{base|mock} are added as optional libraries.

The two java shared libraries do not exist in the pre-P devices.
Therefore, marking the dependencies to the libraries as optional so that
an app is still installable to the old devices.

This is safe because the classes in the java shared libs exist in other
shared libraries (bootclasspath or android.test.runner).

Bug: 113800422
Test: run aapt dump badging on the built GtsBackupHostTestCases

uses-library tags for android.test.base|mock are with
android:required=false, while the same tag for other libs are without
the attribute.

Change-Id: I266c8efb9686e8975be37f31aea0dc22f82370c3