Some apex payload images contain apk files. And these apks need to be
signed during the signing processed when sign_target_files_apks is
called. To support the signing, we can extract the payload and repack
the apex file with the (de)apexer tool. Add the signing support in the
apex_util.
Bug: 146508800
Test: unit tests pass, run sign_apex, sign_target_files_apks
Change-Id: If6d58975248709a144b07dbabf47c27916e5695e
