Synchronize access to our cached account list

* The recent change that moves our account change handling out of the
  UI thread caused ConcurrentModificationException's during account
  creation
* We now synchronize all access to our cached account list to prevent
  this

Bug: 2546724
Change-Id: I92b2becedd36a335a81f7e3bd9617f2923e00e6c
2 files changed