Change envs_lock_ to a ReaderWriterMutex

We were getting a lot of contention on this mutex because it needs to
be held to run through the list of all active environments. Since this
list is only rarely modified we change the lock to a ReaderWriterMutex
and only lock it exclusive when we need to modify it.

Test: Examine systrace of startup with DDMS monitor running.
Bug: 72336488
Change-Id: Ie161b0f9c624384fcf36272c6edb78c4a7164149