crash_dump: clear the signal mask.

crash_dump inherits its signal mask from the thread that forked it,
which always has all of its signals blocked, now that sigchain respects
sa_mask.

Manually clear the signal mask, and reduce the timeout to a
still-generous 2 seconds.

Bug: http://b/38427757
Test: manually inserted sleep in crash_dump
Merged-In: If1c9adb68777b71fb19d9b0f47d6998733ed8f52
Change-Id: If1c9adb68777b71fb19d9b0f47d6998733ed8f52
(cherry picked from commit e740250b)