Removes custom prefix/suffix from MergeDynamicPartitionInfoDicts.

All callers of this function now always pass the same values, so this
change hardcodes those values within the function body.

Fix: 145008064
Test: python -m unittest test_common
Test: build & boot a merged target that uses DAP

Change-Id: I0051c5ba507983231825edfcaf349e574efa451a