miguely fb9092098a Add the ability to hide and expose the Back Layer.
-The exposure and hide actions are animated, and have before/after callbacks.
-Clicks on the content layer when the backlayer is exposed are hijacked to close the back layer.
-The animations do not relayout, by simply sliding the boundaries (bottom/left/right/top).

PiperOrigin-RevId: 160315361
2017-09-21 11:25:14 -04:00
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