PiP: Constrain pinch-resize input to start inside PiP bounds.

Previously, we just check for any two down input event - but they should
both be inside the PiP bounds for the gesture to start.

Bug: 184789307
Test: Manual. Have only one finger inside PiP and one finger outside and
try to pinch, no longer resizes.

Change-Id: Ia2193f8d754423013e98d08399f00e665d353b14