Introduce InputMethodNavButtonFlags

This CL reworks my previous CL [1], which let
InputMethodManagerService report whether the IME switcher icon needs
to be shown or not to the IME process by using IInputMethod IPCs.

It turns out that we need to propagate one more boolean value in order
to address Bug 219820813.  It'd be much clearer if we use bit flags
rather than adding a new boolean parameter to each IPC method.  Thus
this CL rewrites my previous CL by using a bit flag defined in a newly
introduced InputMethodNavButtonFlags.

This is a purely mechanical refactroing.  There should be no behavior
change.

 [1]: I5de9ac0dc8670842edf66306bb4c281c77cea376
      75b935a1

Bug: 215551357
Bug: 219820813
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: I81cb062a08d484ec8ce5d7b2fea64ce19028f82e