Use Android systemTime implementation for host linux.

The Android framework, notably android.view.Choreographer, assumes
that System.nanoTime and SystemClock.uptimeMills return consistent
values. This was true on device, but not on host.

This commit makes those values consistent on host linux. The necessary
support should be in place in kernels 2.6.39 and newer, which have been
available since 2012.

Test: m -j libutils
Change-Id: I833a89a810ae9fb3e8c01f6095ee2aca893c284f