releasetools: Accept PRESIGNED keys in apexkeys.txt.

For an PRESIGNED APEX, it has the following format, which should be
considered as a valid input.

name="foo.apex" public_key="PRESIGNED" private_key="PRESIGNED" container_certificate="PRESIGNED" container_private_key="PRESIGNED"

Bug: 131153746
Test: Run sign_target_files_apks.py on a target_files.zip with PRESIGNED
      APEXes.
Test: python -m unittest sign_target_files_apks
Change-Id: I51076b0c6eddfb75637d37659a08009f0a88e931