[Memory Leak] Clear ViewRootImpl#mFallbackEventHandler's view from ViewRootImpl#doDie()

This ensure calling SurfaceControlViewHost#doRelease() will prevent
leaking view via ViewRootImpl by remote process.

Fix: 427483331
Flag: NONE - released code
Test: manual
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:bce32fd49350a4bcabea4d59dbe0e06d9d5cbbee)
Merged-In: I6e9fce0286b175825c10be30493813c79078ee11
Change-Id: I6e9fce0286b175825c10be30493813c79078ee11