Added android.test.mock and android.test.mock.sdk targets

The android.test.mock will be used for any targets that require
android.test.mock classes but which do not already depend on the
sdk. It is also the library that will be added to the classpath
at runtime when an app has the following in their manifest.
    <uses-library android:name="android.test.mock"/>

The android.test.mock.sdk will be used by targets that require
android.test.mock classes and which build against the SDK. It
contains stub classes (like android.jar). It is also the library
that will be copied into the optional directory of the SDK.

Bug: 30188076
Test: make checkbuild
Change-Id: Ia3fcd47c644b8e5a573f891571a7b3772eac63f9