Temporarily revert back to using file_contexts for tracefs.

An earlier commit moved tracefs file labels from file_contexts to
tracefs.  But this requires a kernel patch that is not present on all
devices, so let's revert it until that is merged.

Bug: 62485981
Test: Built, flashed, and booted two devices.  Verified that the files
have the correct context.  Verified that traceur works.

Change-Id: I8ee3ea9864f73a92943cdbc550131d4a71b842ba