blast: adding a buffer doesn't make the frame dirty

The frame shouldn't be marked as dirty when a new buffer is set.
When a frame is marked as dirty, it forces SurfaceFlinger to do
extra work on the CPU which hurts power/performance.

Bug: 125859110
Test: Take a systrace when running the Blast Chrome.apk. Check that
      there is no extra work done.

Change-Id: I0cf9802f495a7e9f2e5b1f4e58f0a7a4a75d8225