Set stem for uninstalled apex modules

Apex modules with installable: false are not installed, so the
dist rule copies from LOCAL_BUILT_MODULE instead of
LOCAL_INSTALLED_MODULE.  If LOCAL_BUILT_MODULE_STEM is unset,
LOCAL_BUILT_MODULE will have the module name as the stem,
resulting in an incorrect file copied to the dist directory.
Set LOCAL_MODULE_STEM instead of LOCAL_INSTALLED_MODULE_STEM,
which also covers LOCAL_BUILT_MODUL_STEM.

Fixes: 122277867
Test: tapas test1_com.android.tzdata && m dist
Change-Id: I0af0c5adcac478a1ce68c4625203fd9bd56bf94d