crash_dump: defuse our signal handlers earlier.

We have a LOG(FATAL) that can potentially happen before we turn off
SIGABRT. Move the signal handler defusing to the very start of main.

Bug: http://b/77920633
Test: treehugger
Change-Id: I7a2f2a0f2bed16e54467388044eca254102aa6a0
(cherry picked from commit 38ac45df1738496d5581b89845e48daeab7f0219)
1 file changed