Start pinchResize only if pointers are inside PIP

The PipResizeGestureHandler was previusly signaling a start
of the pinch resize without checking that both pointers were
within the PIP bounds region. This caused the TouchState to
be reset with a single tap outside the PIP window while
drag-moving it with another finger, which led to PIP becoming
stuck without snapping to the edges.
With this CL, we make sure that both fingers are inside PIP
before starting resetting the touch state and allowing the
pinch resize to start.

Bug: 188640607
Bug: 186607893
Test: move the PIP with one finger and while still holding it
tap anywhere outside of PIP with another finger - verify you
can still keep moving the PIP with the first finger normally.

Change-Id: I2137180ea1f333fe2e0b67e52d57da5ff9a2d094