Fix cross-thread access to collection in ViewRootImpl

Fixes: 418548559

Flag: EXEMPT Bug Fix

The collection mThreadedRendererViews can be accessed from the
UI thread and the render thread. To avoid multiple threads
modifying and reading the collection simultaneously, a synchronization
lock was added.

Test: new test in ViewRootImplTest
Change-Id: Ie14c9c1b8fae4aeb58e00bf8d0ae65371b296962