Fix accessibilityComponentType crash

[haitaoli]

Motivation

DynamicFromMap internally uses SimplePool object to recycle dynamic prop objects. But the pool is not multi-thread safe. Before accessibilityComponentType prop was added, the only dynamic props are size props such as left, paddingVertical, marginTop and so on. These props are only accessed from the layout thread so the pool works fine. The new accessibilityComponentType prop is accessed from UI thread, so now two threads can access the pool object and caused random errors. This PR make the pool object thread local to avoid synchronization. After this change there are two pool objects created in the process.

Test Plan

Tested RNTester app and didn't notice anything.

Code Review

@lelandrichardson @gpeal

[gpeal]

Way to track this down!