Fix race condition updating local map data.

If the underlying local map changes, it's possible for multiple
threads to try and modify the map data associated with the UnwindLocalMap
object. Add a lock when generating the local map to avoid this problem.

In addition, add a read lock whenever any caller gets the maps iterator.
Updated all iterator callers to make this lock.

Bug: 29387050
Change-Id: Ie34822c3d8fd3bdb3dd126aeeb399969c36508c1