releasetools: Initalize logger when signing.

Other modules have switched to logging module. sign_target_files_apks.py
needs to init the logger to get the logs.

Test: Run `sign_target_files_apks.py -v`. Check outputs.
Test: Run `check_target_files_signatures.py -v`.
Change-Id: Ic68c019f6fb14840561885f1194ad6efdfdb7d82