Don't call dlsym from signal context in signal chain

It is dangerous to call dlsym from within a signal context
since it takes a lock and can lead to a mutex reentry attempt if
timing is bad.

This change adds an initialization function to the signal chain
that calls dlsym for sigaction and sigprocmask from outside the
signal context (from Runtime::Init()).  The results are cached
in a static variable and used from within the signal context if
necessary.

However, tests don't necessarily call Runtime::Init() so we also
need to deal with the case where the signal chain is not initialized
and perform a lazy initialization from inside sigaction or sigprocmask.
This is always outside a signal context since we have not initialized
the runtime.

Bug: 17498571
Change-Id: I59eebfc67cc91c6f1d781a73e5d432ca5269ee48