Don't use APEX stubs between internal libs in the same APEX when

building test_for modules.

This extends the current approach where test modules always depend on
the platform variants of the APEX libs, and only skips the stubs on
them. It still has the limitation that the internal libs must have the
exact same apex_available lists.

Also some improvement of the test accuracy in TestTestFor.

Test: m libartagent-target
  with http://r.android.com/q/topic:libdexfile-noext applied
Bug: 183217299
Change-Id: I2118b8a22c887077867a3ddbbe73437b4a29a6ad