Don't re-render CachedSets

If the GPU is already in the process of rendering this CachedSet, e.g.,
if rendering is falling way behind, then don't try to re-render this
CachedSet because otherwise the system completely falls down

Bug: 187705068
Test: pull down notification shade with blurs enabled very slowly
Change-Id: I396e3a2472dd224a23896ba3186fa3e1ce2345f9