Visualizer: Fix race condition in setCaptureCallBack

Visualizer:setCaptureCallBack needs to make a local
copy of the CaptureThread pointer before temporarily
releasing mCaptureLock. Otherwise, mCaptureThread
can be released in the meantime from Visualizer::release

Bug: 149375271
Test: use EffectsTest app with "Multithreaded Use"
      for the Visualizer test. Repeat releasing the effect
      while capture via callback is active.
Change-Id: Iaea62a0e2f7e7a996c1554b9ed89efb67fbdfc11