build: Add support for PRODUCT_COPY_FILES_OVERRIDES

 * We may have a case where prebuilts need to be built from
   source instead based on an externalized config.
 * New directive takes a list of destination files to remove from
   PRODUCT_COPY_FILES so that other instructions can be supplied.

Change-Id: I7feff16440e54d1676ffddbbd96d5947efa43ede
Remove nulls from the edify generator.

These would break the scripts. WTF are they doing here?

Change-Id: I0c52335a27f7ae974279fde1c2461b2dba24d263