Use System.nanoTime(), avoid desyncing frame times

The rendering pipeline expects frame vsync times as
System.nanoTime(). When the frame vsync time is behind
the most recent frame, the render request is dropped [*].

This reverts the value back to System.nanoTime() which
was accidentally changed in ag/15819081.

* frameworks/base/libs/hwui/renderthread/TimeLord.cpp:46

Bug: 217046739
Test: ???
Change-Id: I21d8737e7acb5ac13bfbcf150006415431d874aa