Use file_contexts.bin from other build to regenerate images.

The old process_file_contexts_bin function did not properly generate a usable
file_contexts.bin to regenerate images, so instead use the file_contexts.bin
from the other partial target files package. When combining any one of several
other partial target files packages with a single system partial target files
package, this file will properly apply contexts as long as the same source is
used for the system partial target files.

Test: Verify that file contexts are properlty applied to vendor image.
Bug: 131584454
Change-Id: I16f8cc3b7f2eb7f09746f0ddcb2c1daf3fd19da6