Build merged apexkeys.txt/apkcerts.txt by partition.

Propagate partition tag data to apexkeys.txt and
apkcerts.txt so that merge_target_files.py can build
merged versions of these files by filtering the
framework files for framework partitions and filtering
the vendor files for vendor partitions.

Bug: 138942268
Change-Id: Ic3226728e97dae84d38ec230ccc86d1b124bea94