Clean up back handler from prototracer

Bug: 160161606
Test: Switch darkmode serveral times and verify there aren't
      old refs in the heap dump
Change-Id: I1678d10e725f6bdcbc807064413b4bc549ef8ef2
Merged-In: I1678d10e725f6bdcbc807064413b4bc549ef8ef2
(cherry picked from commit 469a3084b77d506460ec3a0c070e1a32f5c878eb)
1 file changed