Support IME switcher icon visibility update

With this CL, the IME switcher icon becomes visible only when
necessary, even if InputMethodService renders the back button and the
IME switcher button in the gestural navigation mode.

Implementation idea:

InputMethodManagerService#shouldShowImeSwitcherLocked() is the source
of truth about whether the IME switcher visibility should be shown or
not, and it internally depends on the following conditions:

 A. com.android.internal.R.bool.show_ongoing_ime_switcher
 B. Whether the IME switcher is already shown or not.
 C. Whether the IME is perceptible or not.
 D. Whether one or more hardware keyboards are attached or not.
 E. Keyguard state.
 F. What IMEs and their subtypes are enabled.

Here are what those conditions would mean for this project.

 * A is considered to be a per-device constant value.
 * B, D, and F can happen at any time outside of the IME lifecycle
   events such as startInput().
 * C is no longer relevant if those buttons are rendered by the IME.
 * E is considered to be constant throughout each startInput() cycle.

This CL uses the following 3 IPCs to notify when the IME switcher
visibility is changing.

 1. IInputMethod#initializeInternal()
 2. IInputMethod#startInput()
 3. IInputMethod#onShouldShowImeSwitcherWhenImeIsShownChanged()

1 and 2 will be used to provide the "initial" value to avoid potential
flickers.  3 is still necessary to take care of async changes
triggered by B, D, and F.

Fix: 215551357
Test: Manually verified with for the following scenarios:
 * Enabling/disabling multiple IMEs
 * Attaching/detaching a hardware keyboard
 * Showing/hinding the IME switcher
 * Showing an IME on the lock screen
Change-Id: I5de9ac0dc8670842edf66306bb4c281c77cea376