Add main thread and reentrant asserts to chase down crashes

We're seeing crashes due to view hierarchy violations that shouldn't be
possible. Adding some guards to make sure we aren't running into
off-thread hierarchy manipulation or re-entrant calls to the update
code.

Test: manual
Bug: 135018709
Change-Id: I4b1f2bd7e3a6f80384486d59b9f56fc3713537cf