NetworkStats: Fix race condition causing system server crashes

NetworkStatsService uses an internal boolean to know when it has
started for the purpose of preventing access to other internal
variables before they are initialized.

However that boolean is set to true in systemReady() non-atomically
with respect to the initialization of the other variables it guards,
which can cause the system server to crash.

This patch fixes this concurrency bug by moving setting the internal
boolean flag and the variable it guards in one atomic synchronized
block.

This patch also removes code checking if bandwidth control is enabled,
because this is now always true.

Bug: 132767673
Test: Compiled.
Change-Id: Ia089b5767ce271d669879c975508654d4dd03429