StrictMode: optimize common case (just dropboxing)

The way StrictMode is used during development, just dropboxing
violations, could be a little more optimal, taking the
ActivityManagerService call off the main thread.  But we can only do
this safely in the case where that's the only penalty.

Data suggests this call, despite being async, still takes around 30
milliseconds.  This isn't a major win, and arguably it might be a
_better_ idea to slow down people's event loops more and further jank
up their animations on violations, but I thought any less overhead
from StrictMode, the better.

Change-Id: Iad9cce1cb4a084fa64abc4b5e1b4f3bff6a08c94