Stem names are used in apkcerts.txt

_apkcerts_write_line has assumed that the stem name of a package is the
same as the module of it. That assumption however breaks for
APK-in-APEX, in which case the stem name is Foo while the module name is
Foo.com.android.bar (where com.android.bar is the name of the APEX where
the APK is in).

Fixing the issue by recording the stem name and use it.

Bug: 155440232
Test: OVERRIDE_TARGET_FLATTEN_APEX=true m apkcerts-list
and check the generat4ed apkcerts txt file to see that it has
Tethering.apk instead of Tethering.com.android.tethering.apex

Change-Id: I56bc55e2231eb946617a9d6f97d55c9b61f3bff7