Defer important for a11y event

This is a quick fix to avoid RV a11y importance change events during
a layout. These events receive an instant callback from the a11y service
which cannot be handled properly because view is in transition from
hidden to visible.

Bug: 30787618
Test: android.support.v7.widget.RecyclerViewAccessibilityLifecycleTest
Change-Id: I65d5aa0f73b0e6433deb617c7b413f276217dd98
3 files changed