Include partition tag in apexkeys.txt.

The partition tag helps merge_target_files.py determine
which apexkeys.txt to select from the framework partial
target files and which to select from the vendor partial
target files. The partition tag is the pysical partition
name, for example, a system_ext module on a device where
system_ext is at system/system_ext has a system
partition tag.

Bug: 138942268
Change-Id: Ia07887b34f1aa77dae94ef23610dfef83c1a5849