Add Null check when excluding IME window surface for snapshot

TaskSnapshotController.createTaskSnapshot crashes when initializing
SurfaceControl$LayerCaptureArgs for excluding NULL IME window surface
control.

As SurfaceControl#nativeCaptureLayers specified the elements in
excludeLayers array should be non-null,

Add a null check before adding excludeLayers when calling
SurfaceControl.captureLayersExcluding to fix the crash issue.

Fix: 174504297
Test: atest WmTests:TaskSnapshotControllerTest#\
      testCreateTaskSnapshotWithExcludingIme

Change-Id: Ie6fa7aa4ac61fc49a6235882d700e1e4d8437666