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
Original-Change: https://android-review.googlesource.com/1765686
Merged-In: Ie7694a63f5203ee7c83830ca13d97219b7949fd7
Change-Id: Ie7694a63f5203ee7c83830ca13d97219b7949fd7