DO NOT MERGE ANYWHERE unlock_wave: replace with black

The unlock_wave asset is used by an internal widget that does not appear
to be used by Android Wear. This asset can be removed to save sapce on
the system partition. It is replaced with black PNGs in the event that
it is loaded and something in frameworks expects these files to be
present.

The black replacements are just a few kB compared to the hundreds of kB
of the original copies.

Bug: 28215770
Change-Id: Ie70f5013c48dbe0bde25992c64ca5023ae37b604