Install the ota keys under recovery as a zipfile

Recovery can now parse the pem encoded x509 keys from a zipfile. So
instead of dumping the keys into a text file with some intermediate format,
we can simply create a zipfile with the keys.

Bug: 116655889
Test: make bootimage and check the generated zipfile, run sign_target_files_apks
Change-Id: Ib76feecfb26d6be713a07644e80ec96133759004