surfaceflinger: Use only one EventThread when phase offsets are same.

SurfaceFlinger uses two event-threads to support phase-offsets
for application and SurfaceFlinger itself. Extra thread may
cause scheduling issues and wake-up latency for either
SurfaceFlinger or Application, depending on scheduling, even
when running at higher priority as these threads have low-load
and qualify for task-packing.

Use single event-thread when phase offsets are same.

Debug runtime service call behavior change when same phase-offset:
1018 (set phase offset for Application): will change both
Application and SurfaceFlinger phase offsets.
1019 (set phase offset for SurfaceFlinger): Will be a no-op.

Change-Id: I2514b4ff6249e9bce3eb19f60b1dfe0de12b86d9