Renames --gki_signing_extra_args to --gki_signing_signature_args

This is to avoid confusion with --gki_signing_extra_args from
build/tools/releasetools/sign_target_files_apks.py, which adds
extra arguments into the original --gki_signing_signature_args
for mkbootimg.

Bug: 177862434
Test: atest --host mkbootimg_test
Change-Id: Idb9b4003120950947da021f73d0e3f416230c75f