This change is needed to introduce new theme easily. As a starter this
change introduces Keyboard.popupHintIcon attribute and
Key.keyLabelOption.popupHint flag to show popup hint icon that is
previously specified by Key.keyHintIcon attribute.

Bug: 4436327
Change-Id: I6b1c5c1b88eece541141b2d2cedd164ee02e5c2a