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
fa17afef53