Refactor in preparation for removing android.test.mock

The goal of this change is two fold:
1) Allow the test-runner/src classes to continue to be included in the
   JavaDoc even when they are removed from the Android API.

2) Make it easy to remove android.test.mock classes from the API.

This was done in two stages:
1) Push the reference to test-runner/src from fwbase_dirs_to_document
   down to dirs_to_document and
   framework_docs_LOCAL_API_CHECK_SRC_FILES.

2) Expand the reference in framework_docs_LOCAL_API_CHECK_SRC_FILES into
   two new variables that reference the classes from test-runner/src.
   One references those that are required for android.test.mock and the
   other references the remaining ones.

Bug: 30188076
Test: make checkbuild
Change-Id: I558164e6c9ea2a24bd58e5f4f54dd4c3b2a11924