[Magnifier-87] Fix deadlock causing ANR

Before this CL, the magnifier could deadlock when the following
happened:
1. the renderer is asked to draw (and a frame callback is provided)
2. a #dismiss() happens on the UI thread. This acquires mDestroyLock
(previously line 309)
3. InternalPopupWindow#destroy() is called, and this calls
mRenderer.destroy(). This attempts to destroy the renderer on the UI
thread, however the UI thread will wait until the pending frame callback
corresponding to step 1 is executed on the render thread.
4. The frame callback starts executing on the render thread, and tries
to acquire mDestroyLock (previously line 1093). However, this is held by
the UI thread, so a deadlock happens.

This CL completely removes mDestroyLock, relying on the existing
synchronization between the UI and render threads described in step 3.

Bug: 134584742
Test: manual testing
Change-Id: Ia4c75b5b997e0ed94d5a3814dd4507a8fffa124d