SF: introduce debug.sf.hwc.min.duration

debug.sf.hwc.min.duration can be set for specific devices to
indicate the shortest duration hwc will take to present a frame.
This value is used by SurfaceFlinger to calculate the
earlistPresentTime used for preventing an early present.

Bug: 190842189
Bug: 188854955
Test: SF unit tests
Test: run TouchLatency with debug.sf.hwc.min.duration set to a value
Change-Id: I8ffe92e5e6b355892c6bb073229f417fb0e9588c