Allow devices to extend the system manifest.

Introduces DEVICE_FRAMEWORK_MANIFEST_FILE, a list
of files which are added to system/manifest.xml.
This is required for devices to properly display
what hals they implement and also therefore for
them to pass vts_treble_vintf_test since it
now tests manifests based on hal origin.

Notice, this is named singularly to match
DEVICE_MANIFEST_FILE which is also a list of files.
They may be better both named "FILES", but for
consistency and legacy reasons, they can be thought
of as "everything that composes the X manifest
file".

Fixes: 70042049
Test: add system manifest extension which needs it
  and it no longer fails vts_treble_vintf_test for
  hals being served from the wrong partitions.

Change-Id: I1f59d5c3cadb7a7d4576b73196ca7b41103a49c5