Forward 'help' and '-h' to default implementation for ShellCommand.

This makes 'adb shell settings help' and 'adb shell settings -h' work.

Previously:
 $ adb shell settings help
 Invalid command: help

Updated:
 $ adb shell settings help
 Settings provider (settings) commands:
   help
       Print this help text.
   get [--user <USER_ID> | current] NAMESPACE KEY
 ...

Test: adb tested on local devices.

bug: 113100523

Change-Id: Ifc3f541aa12da9c79b9b7e9db0e30d4aace3392a