Make Log.isLogagble @FastNative

This CL doens't change println_native() because:
- To avoid potential risks (jank?) because it's kind of late in the RVC
cycle.
- The JNI overhead is unlikely to be a major problem in logging. If apps
are making *that* many log calls, that itself would be a bigger
problem.

Test: treehugger / boot
Bug: 152217649
Change-Id: I86aeb62b217e5331e6bbd02a0ba592fd050a41b2