Use IBinder#shellCommand() for 'adb shell ime'

This is a preparation CL to add a new command to 'adb shell ime'.

Currently 'ime' command is written in Java language that relies directly
on the internal Binder IPC interface IInputMethodManager.

This is not ideal because:

 1. We have to keep maintaining IInputMethodManager methods used
    only by the 'ime' command.
 2. Adding new options to the 'ime' command is tedious when it
    requires new methods in IInputMethodManager.

With this CL, all features of 'ime' command are re-implemented inside
InputMethodManagerService (IMMS) on top of Binder's "shell command"
feature [1].  Like 'am' command was gone recently [2], now 'ime' command
is also a simple shell wrapper to forward options to 'cmd input_method',
which allows us to 1) reduce the code duplication and 2) give non-zero
status code when the command fails with Java exception.

 [1]: I76518ea6719d1d08a8ad8722a059c7f5fd86813a
      9461b6f9
 [2]: Ia8187196af597046fd2e7092dbf19ce1dc1ea457
      1704e3cf

Bug: 70475949
Test: adb shell ime
Test: adb shell ime help
Test: adb shell ime dump
Test: adb shell ime list -a
Test: adb shell cmd input_method
Test: adb shell cmd input_method help
Test: adb shell cmd input_method dump
Test: adb shell cmd input_method list -a
Change-Id: I9a2dbbf1d4494addbe22c82e2c416eedc4d585f2