Don't update quota for non-emulated volumes.

Since they don't support quota. Also, don't throw an exception if we
can't find a volume for the passed-in path, but warn instead: the volume
may be unstable and have disappeared by the time we do this.

Bug: 152618535
Test: boot emulated image; verify no crashes when inserting media on
      public storage
Change-Id: I0e6b8f5c9f9f35b80b7bd5f6255a70e210a30474