Fix InsetsConsumer leak

Activity was still referred from ImeInsetsSourceConsumer after
ViewRootImpl's mView was destroyed when ViewRoot's surface is cleared
using die signal.
This CL makes sure we still free-up resources at die signal.

Fix: 157955883
Test: atest NexusLauncherTests
Change-Id: Ia48f7b7a8cf6b867ce75b2b7393a60ba73b0c3d0