Bug 1013570 - [B2G][Keyboard]Double or triple tapping on the spacebar

can result in 2-3 periods, as well as deleted characters.
 - Send the next key after the previous sendKey is resolved.
So I was confused with `handler` v.s. `inputSequencePromise` because we return the promise in the `click()` method. I checked `keyboard.js` and we don't make use of it, so I think we shouldn't return a promise at all.

Other than that, I think we need to discuss what `click()` actually waits.