Fix mMobileIfaces is not protected by lock

Currently, mMobileIfaces is accessed from multiple threads, and
should be protected from concurrent accessing. However, since the
variable could be accessed frequently, holding the mStatsLock
would make this be blocked by network stats I/O operations.
Thus, protect the variable by making it volatile.

Test: Wifi on/off stress test
Bug: 192758557
Change-Id: Ie7694a63f5203ee7c83830ca13d97219b7949fd7