Add synchronized block when accessing global variables

This is catched by errorprone where global variables accessing
is not protected by the declared lock, which could lead to a
potiential race problem where these variables are changed but
cannot be seen for the invocation.

This is also safe since:
  1. The method called inside mDefaultNetworks is already holding
     the same lock.
  2. Multiple global variables are protected by this synchronized
     block.

Test: TH
Fix: 181642673
Change-Id: I58e7f124de1f0291e9323ab0b9cf8f52cec32818