releasetools: Store the proper base64 release key in the target files

This code was dropped when the original change
(I7528a8e7c484ea9209cd665b9263328ae834586a) was cherry-picked into 13.0.
Add it back in to the sign_target_files_apks.py file.

Change-Id: I0177a7d2a39ae03b21d0b2b4ad48a621ca5afd7b
Ticket: CYNGNOS-2067
(cherry picked from commit 78479274)