Fix NPE in letterbox double tap

We check the Letterbox surrfaces are in a valid state before invoking
the double tap callback. We also put the callback invocation under the
WindowManagerService GlobalLock.

Fixes: 294185001
Test: Run `atest WmTests:LetterboxTest`

Change-Id: I076e0b62b3567145f1185e29a29411f53bdf5b90