Add trace event for fallback composition

When the HWC rejects composition of any layer, it is returned to
surfaceflinger and queued to the GPU for doing client composition.
This event is important to track as it can show why the GPU is busy at
that time.

Test: Set fallback composition in the middle of a GAPID trace from the
commandline
Bug: 140236090

Change-Id: Id1cb71f301023743dc9fa6a1396f8a525b5de7bd