Don't reset LOCAL_*MODULE_STEM.

By resetting we lose LOCAL_*MODULE_STEM values for the second include
of the *_internal.mk.
We don't need to reset because we now store the default value to a
temporary variable my_*module_stem in configure_module_stem.mk.

Bug: 27775991

(cherry picked from commit c160c3f2)

Change-Id: I5d9b7f7214d98a1bf953fadb9951a8af1e993057