Notice support for APEX

Notice file for an APEX is created by merging notice files for the
modules included in it (plus the notice file for the APEX itself if
specified).

Notice files having the same content are not duplicated; it is emitted
only once.

Bug: 128701495
Test: m (apex_test is amended)
Test: m and inspect $(PRODUCT_OUT)/obj/NOTICE.txt to check there are
license entries for /system/apex/*.apex files

Change-Id: I169d91038291a6c71615de97cf5b03174afab5d4