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
Change-Id: I36d1ff3d177818677fb38bdb3d340dd2ff34c404