Do not synchronize boolean reads/writes

This patch removes the synchronization around the private variable
mRunning inside of IpReachabilityMonitor and instead qualifeis the field
as volatile.

Synchronization is not needed for reads/writes on native fields or
object references because they are already guaranteed to be atomic.

Synchronization here was used for enforcing memory visibility across
concurrent threads indirectly through monitor acquire/release.
The volatile keyword achieves this in a more explicit way.

Also, this patch changes the way that probeAll() copies the
IpReachabilityMonitor's mIpWatchList by temporary holding mIpWatchList
keys into an ArrayList instead of a more expensive HashSet. Since Java
HashSet are just degenerated HashMaps, and that key iteration order is
based on key hash, the iteration order over this temporary collection
will be consistent for the same mIpWatchList.

Test: refactoring CL. Existing unit tests still pass.
Change-Id: I86ca6d54cb41ec78281e224a8d8ffd7155451132