Security Fix: Race Condition + NPE

ShellSubscriber is lazily initialized, and multiple threads can attempt
to write the same pointer since it is not initialized in threadsafe
code. Additionally, there is an NPE that crashes statsd when a null
ResultReceiver is passed in, which allows an attacker to repeatedly
crash statsd until the race condition occurs. More details, including a
proof of concept attack, are in the bug.

Bug: 141243101
Test: repro steps in bug no longer crash statsd
Test: with only the lock on iniitiallizing mShellSubscriber, statsd
still crashed but after ~7 minutes, no race condition occurred.

Change-Id: Ib56f888620497fb41d1627c07867693eb251738e