EGL: Enable eglGetFrameTimestamps.

Also clean up the specification description:
1) Reverse order of revision history notes.
2) Specifically mention polling from another thread.
3) Make it explicit that eglQueryTimestampSupportedANDROID
   works for the timestamps of both eglGetCompositorTimingANDROID
   and eglGetFrameTimestampsANDROID.

Test: adb shell /data/nativetest/libgui_test/libgui_test
    --gtest_filter=*GetFrameTimestamps*

Change-Id: I26ff1ea97654d8953a18afaad1455813191abe7d