fixed coalescing key generation for RCTTouchEvent
fixed coalescing key generation for RCTTouchEvent

Summary:Turns our using the same coalescing key until a person removes all fingers off screen is not ideal.
It doesn't work in a case where the first finger starts moving on screen and then a second finger joins it later (almost any pinch gesture),
since we would try to coalesce move events from the start when only one finger was touching screen with events where two fingers were moving on screen.
That doesn't work and results in a crash.

I've changed the logic for generating the coalescing key in order to prevent this.
We no longer have a single key for a single gesture, but we change the key each time amount of fingers increases ("touchStart") or decreases ("touchEnd").

Reviewed By: javache

Differential Revision: D3138275

fb-gh-sync-id: c32230ba401819fe3a70d1752b286d849520be89
fbshipit-source-id: c32230ba401819fe3a70d1752b286d849520be89

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