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
Also fixes small nit from previous change to write_sorted_data().
Bug: 132788610
Test: python -m unittest test_merge_target_files
Test: Creating and booting a merged build.
Change-Id: I3dc43a4fe55b86b436dec08feb5d70096d38de36