Simplify IMMS#onCommandWithSystemIdentity()

This is mechanical refacotring.  There should be no
developer-observable behavior change.

This is a preparation to fix Bug 180765389.

Bug: 180765389
Test: "adb shell ime" still shows help.
Test: "adb shell ime help" still works.
Test: "adb shell ime list" still works.
Test: "adb shell cmd input_method" still shows help.
Test: "adb shell cmd input_method dump" still works.
Test: "adb shell cmd input_method help" still works.
Test: "adb shell cmd input_method ime list" still works.
Change-Id: I2fb5d9d84d9e51890e432e996ca294778d507362