signal_catcher: Only notify tombstoned when we're using it.

This would otherwise cause a benign warning message when tombstoned
based stack dumping was turned off. The notify would fail immediately
with an EPERM due to SELinux restrictions.

Bug: 62779030
Test: test-art-host
Change-Id: Icc92cf0ebf146091317bb48b9968e20e9ab99da5