sigchain: properly block signal when we forward.

The previous code incorrectly chose to unblock the received signal when
SA_NODEFER was passed, instead of blocking the signal when SA_NODEFER
was not passed.

Bug: http://b/38305752
Bug: http://b/62189102
Bug: http://b/62202711
Test: m test-art-host
Change-Id: I2a7176b2a9fc7cdd0193face46f6d14a617ca814