Fix extension stripping

[dflock]

The existing code:

target_apk_unpacked = target_apk.rstrip(".apk")

will remove any of the characters in the set .apk from the right side of the string. For example, if target_apk is my_app_apk.apk, then target_apk_unpacked will end up as just my_app_. To remove only the extension from the target_apk path, do this instead:

target_apk_unpacked = os.path.splitext(target_apk)[0]

[ryanking13]

Thanks @dflock!