sigchain: limit use of SetHandlingSignal.

The native bridge and user signal handlers are able to not return to
our signal handler, leaving the HandlingSignal flag set to true for the
rest of the lifetime of the thread. Fix this by only using
SetHandlingSignal for handlers that we know will return (i.e. the ART
fault handler).

This effectively reverts commit 90444558, which means sigprocmask's
behavior is back to filtering out claimed signals when inside a user
signal handler.

Include an update to test/115-native-bridge from Zhenhua Wang, to make
sure we keep handling signals when a signal handler longjmps away
instead of returning.

Bug: http://b/37988407
Test: m test-art-host
Test: m test-art-target
Change-Id: Ia7159ddfa38f1f055e5cd6089c849a208d335752