License info for APEXes are correctly gathered

This change fixes a bug that license info for non-flattened APEXes are
not captured in /system/etc/NOTICE.xml.gz file. For non-flatted APEXes,
we have been creating NOTICE.html.gz file by concatenating all the
license infos of the modules that contributes to the APEX and embedding
the file into the asset directory of the APEX. Then at runtime, the info
is shown through the "Google Play System Update Licenses" UI. However,
this was problematic because the UI only shows license info for the
Google-signed APEXes, leaving OEM-signed APEXes (a.k.a. optional
modules).

The problem is now fixed by associating a merged license file with each
APEX and exporting them to Make, so that the merged license files are
included in the partition level /system/etc/NOTICE.xml.gz file
regardless of whether the APEX is a Google-signed one or not.

This also fixes a bug that license info entries are created for the
runtime paths /apex/<apex_name>/<path_to_a_file>, which is not necessary
as they are already included in the license info of the containing APEX.

Bug: N/A
Test: Go to Settings->About Phone->Legal information and check
that a) /system/apex/*.apex files are shown and b) /apex/<apex_name>/*
files are not shown

Change-Id: I2c25c803b6a4c39b24bb3f724502699382fab50c