Separate android.test.mock from test-runner source

Extracts the source for the android.test.mock library from the
frameworks/base/test-runner directory into its own
frameworks/base/test-mock directory. They are already treated separately
at runtime and compile time so this just makes the separation complete.

Bug: 30188076
Test: make checkbuild
Change-Id: I20e5b06ba79677e76117c82e9f9e2ecd15e5fed6