Move notifyUserAction() to IInputMethodPrivilegedOperations

This CL re-implements the way to propagate user action on an IME to
InputMethodManagerService (IMMS) so that we can dynamically update IME
Subtype rotation list discussed as requested in Bug 7043015.

It turns out that my previous CLs [1][2][3][4] are unnecessarily
complex because I tried to monitor user behavior in the IME client
process rather than in the IME process.  In the end, I ended up
introducing a sequence number protocol for the sake of performance
with a ton of complexity.

This could have been implemented in a much safer and simpler way by
sending user action signals from the IME process to IMMS, because

 A. IME already knows when it switches to a new subtype. IME needs to
    send a signal only once per subtype change.  There is no need to
    use sequence counter.
 B. Malicious IME client is unable to disturb IME rotation list by
    sending a fake signal because the IPC endpoint is no longer exposed
    to IME client processes.

In case there remain some applitations that still call this hidden API
via reflection without gracefully handling exceptions, this CL keeps
InputMethodManager.notifyUserAction() as a stub method so as not to
break such applications.

 [1]: I11ed9a767588f8080753cd9bce011dac7db579ad
      d7443c83
 [2]: I7f3e13a7226ef0dceee82b67e8a0d8536f7e9807
      2a6a8d2fbb063c84e388c185402c4ca788618c72
 [3]: I19ad8542659bc092b92ee13eb9f1d68ddd4b815a
      b56c6c721fc01fba8e36632d8e28f5123831abc5
 [4]: I03fa436df0a5e348b3f93170aab3a8ac5a7e1677
      c21ccc151631663d71230a3c1c756d94b575ab9e

Bug: 113177698
Fix: 114159783
Test: Manually verified as follows
  1. Build and flush aosp_taimen-userdebug
  2. make -j SoftKeyboard
  3. adb install -r $OUT/system/app/SoftKeyboard/SoftKeyboard.apk
  4. adb shell ime enable com.example.android.softkeyboard/.SoftKeyboard
  5. Open AOSP Keyboard settings
  6. Enable "English (US)", "French", and "German"
  7. Open SoftKeyboard settings
  8. Enable "English (United States)", "English (GB)"
  9. Open the Dialer app and tap the top edit field.
 10. Make sure that the IME layout rotation order when tapping the
     globe key will be updated only when you tap the keyboard to enter
     some character.
 11. Also confirm it with "adb shell dumpsys input_method" by checking
     "mSwitchingController:" section there.
Change-Id: Icc1f9c7f530f0144ecfd460e86114e109ae0044e