blast: flush transaction wake ups

When SurfaceFlinger's main thread wakes up just to flush
transactions and no transactions are ready to be flushed, go back
to sleep.

Bug: 128520233
Test: Play a 30hz video that has desiredPresentTimes. Check that
    SurfaceFlinger is only doing a full present cycle 30 times
    a second.

Change-Id: Iae11f9a604a3ea5b0d553947f47133871e608fca