Fix potential deadlock in unregisterStaleHandlers()

The scenario is that a call to unregisterStaleHandlers() is in progress,
and is holding a temporary sp<ALooper> reference to an active ALooper inside
of the loop. At this point the only other remaining external reference to
the ALooper goes away, so the temporary sp<ALooper> in the loop is now
the only reference keeping that object alive. When the loop iterates and
the sp<> goes out of scope, the ALooper destructor is called, which in turn
calls unregisterStaleHandlers again, resulting in a recursive lock.

Bug: 17300093
Change-Id: I116f2ffab4ae7c43b6bcf54a367ae6f9d77c9626