Map "shell ime" to "shell cmd input_method ime"

This is a follow up CL to my following CLs:
 * Use IBinder#shellCommand() for 'adb shell ime' [1]
 * Add a shell command to reset selected/enabled IMEs [2]

Previously
  adb shell ime <command>
was an alias of
  adb shell cmd input_method <command>

With this CL,
  adb shell ime <command>
is now an alias of
  adb shell cmd input_method ime <command>

This allows us to rename a sub command "reset-ime" to "reset" without
worrying about ambiguity about what will be reset.

Note also that the built-in "dump" command now works only as
  shell cmd input_method dump

 [1]: I9a2dbbf1d4494addbe22c82e2c416eedc4d585f2
      926488d7
 [2]: I1e27aa1c9edf5d9a6d529c469ad42fd1e42b13b1
      cc97ebd0

Bug: 70475949
Fixes: 70731324
Test: Manually tested as follows.
  1. make -j SoftKeyboard
  2. adb install -r $OUT/system/app/SoftKeyboard/SoftKeyboard.apk
  3. adb shell ime enable com.example.android.softkeyboard/.SoftKeyboard
  4. adb shell ime set com.example.android.softkeyboard/.SoftKeyboard
  5. Make sure that SoftKeyboard is enabled and selected
  6. adb shell ime reset
  7. Make sure that SoftKeyboard is no longer enabled and selected
Change-Id: I2a325dcd6108e357420d47880e4cbd9f81670ec9