Add IMMS#showInputMethodPickerFromSystem.

Since the NavBar will be supported for multi-display, for
single session IME, it will be possible that IME switcher icon
will shown on external display.

Add IMMS#showInputMethodPickerFromSystem for system modules (i.e.
Settings or SystemUI) to pass displayId for creating right display
context, so that IME switcher dialog can shown on the display
correctly.

Also Add a TODO item for ACTION_SHOW_INPUT_METHOD_PICKER notification
that currently only support showing IME picker for default display,
this should support per display after supporting status bar per display.

Bug: 119933861
Fix: 120050928
Test: manual, verify IME switcher dialog can shown on external display
      when tapping IME switcher icon on external navbar.
Test: atest InputMethodManagerTest#testShowInputMethodPicker

Change-Id: Ic7d7c5a7ad8005a3fbd9d1c1b73e3c5a39a07001