Make snapshot surface relative to parent

- When in splitscreen and the snapshot aspect ratio matches the parent
  we need to offset to ensure that the calculated matrix is relative
  to the parent otherwise

Change-Id: I2fcd72bde40171fb456108993214b00741abe7fd
Bug: 157624680
Test: Enable waterfall/tall cutouts, quickswitch in both full and
      splitscreen
Test: atest TaskSnapshotSurfaceTest

services/core/java/com/android/server/wm/TaskSnapshotSurface.java

@@ -391,6 +391,7 @@ class TaskSnapshotSurface implements StartingSurface {
             frame = null;
             mTmpSnapshotSize.set(0, 0, buffer.getWidth(), buffer.getHeight());
             mTmpDstFrame.set(mFrame);
+            mTmpDstFrame.offsetTo(0, 0);
         }
 
         // Scale the mismatch dimensions to fill the task bounds