Correctly gather boot jars from APEXes

Jars in APEXes have Make module names <jar_name>.<apex_name>. e.g.
updatable-apex.com.android.media. Previously, we have used <jar_name>
which actually meant the platform variant of the jar. This is not only
incorrect, but also is causing problem as the platform variant is no
longer available when the jar is configured to be available only for the
corresponding APEX (via the apex_available property).

Fixing the problem by correctly using <jar_name>.<apex_name> scheme.

Bug: N/A
Test: m

Change-Id: I6e255ce88c9bd80120b29197fb2637a64010f531
Merged-In: I6e255ce88c9bd80120b29197fb2637a64010f531