releasetools: Fix an issue when using APEX container key mapping.

The keys_info in the touched code is a tuple, which is immutable.

Bug: 123716522
Test: Run sign_target_files_apks.py with '-e foo.apex=bar' that replaces
      the APEX container key.

Change-Id: I4e57e46c93a56b7f6646764d021ebb42c19bf7f5