Remove mAbortKey flag completely

These three conditions assign true to the mAbortKey flag.
1. setKeyboard() sets the new keyboard layout.
2. long press shows the pop-up mini keyboard.
3. ACTION_CANCEL event is occurred.

In the case 1, in LatinIME so far, only "symbol key" and "language
switch" cause keyboard layout change.  In both cases, the event is
occurred in onUpEvent(), so we do not need to worry about aborting
event.

In the case 2, long press is used only to pop-up mini-keyboard and at that
time mMiniKeyboardOnScreen could be used as a flag.

In the case 3, though I'm not sure when this case occurs, in
onCancelEvent() all the things that should be canceled are canceled
in onCancelEvent().

Bug: 2910379
Change-Id: I12377c5f3e808f1c017ce980e12b1513895047bc