Preemptively create Bitmap from GraphicBuffer

- This call moves up the code to create a ThumbnailData from the
  TaskSnapshot to ensure that it is registered with the GC even if there
  is no active TaskView bound to process the event.

Bug: 62360156
Test: Run "bit CtsMediaTestCases:.DecodeAccuracyTest", and ensure that
      the "Lost RAM" is relatively unchanged afterwards

Change-Id: I64af55aa40e80604864e17045798823cc6fee483