Add LOCAL_VENDOR_MODULE as a synonym to LOCAL_PROPRIETARY_MODULE

The vendor image will have more than just proprietary modules in it
under Treble, so let's stop marking open source code as proprietary just
to move it to vendor.

Bug: 36452052
Test: compare build-aosp_arm.ninja before/after, no changes.
Test: Try defining LOCAL_VENDOR_MODULE / LOCAL_PROPRIETARY_MODULE in the
same and different ways, ensure it works.
Change-Id: I0ab046bcb7563a20d44ed00667e18497ef0873a8