StrictMode: dropbox VM-wide violations (e.g. CloseGuard) async

To be clear, the dropbox violations were already async in the
ActivityManager, but the Binder call was often 30 ms anyway.

This optimization was already done for per-thread violations earlier,
but was never done for VM-wide violations because they weren't common,
until CloseGuard came about.  Now that CloseGuard fires a lot, apply
the same optimization to VM-wide violations.

This CL also addresses a concern of Dianne's earlier of too many
threads being outstanding.  So now there's a paranoia check with an
upper bound on how many outstanding ActivityManager calls are
in-flight.

Change-Id: I95e0816105ab862f0f241052b149c9a46a70ce9c