Fix locking on string init map (again).

Fix locking on string init map (again).

Follow-up to
    https://android-review.googlesource.com/172036 ,
    https://android-review.googlesource.com/171621 .

Don't overwrite existing values, only insert new ones.
(Also improve performance by using move semantics.)

This prevents the following race: Thread 1 looks for string
init map for a method but doesn't find it, so it starts to
construct a new map. Thread 2 is doing the same but it's
faster and actually inserts the new map and keeps a pointer
to it. After Thread 2 releases the lock, Thread 1 acquires
it and starts to Overwrite() the element that the Thread 2
is currently using, so Thread 2 ends up looking at a map
that's being actively modified.

Change-Id: I135571af644363ea7bb282969a1bc7287b34f9b2