Move file labeling to genfs_contexts.

This should slightly improve performance, as file_contexts is slower
than genfs_contexts.

Now that the kernel patch enabling genfs labeling of tracefs has
landed, we can re-enable this.

Bug: 62413700
Test: Built, flashed, and booted two devices.  Verified that all of
the files have the correct context and that wifi, camera, and traceur
work.

Change-Id: Ifc1c6ac634b94e060ed1f311049bd37f6fcc8313