Add Null check for mView.findFocus() in ViewRootImpl

Some apps like Netflix crashed while handleWindowFocusChanged but mView
somehow has became null.

It's fair to check null for mView.findFocus() since other places
in ViewRootImpl that calling this method also add null check.

Fix: 153862451
Test: atest CtsInputMethodTestCases

Change-Id: I86bd5f3193deabe28bb425a8564017f7b32b90f3