Bug 963096 - Caps lock is switched on when quickly tapping shift before
and after another key.
 - Maintain only one target for double tap, so clicking another target
   would clear the previous one.
Merge pull request #25697 from RudyLu/keyboard/Bug963096-interrupt_doubletap_timer

Bug 963096 - Caps lock is switched on when quickly tapping shift before and
r=timdream