SF: Register layers on first strong reference

In ag/9549429, LayerHistory stores a wp<Layer> on Layer construction, at
which point RefBase::mStrong is not yet 1. Since wp<T>::promote succeeds
in that case, LayerHistory may concurrently create a sp<Layer> with sole
ownership on the main thread while checking for expired weak references,
resulting in premature layer destruction.

Bug: 144218964
Test: None (sporadic crash)
Change-Id: Ia7c89a090920588b216b719fce36c9f64a126e81