crash_dump: switch to PTRACE_SEIZE.

ptrace(PTRACE_ATTACH) does not immediately put the traced process in a
stopped state. If we manage to call PTRACE_CONT on it before it
finishes, we'll fail. Switch to using PTRACE_SEIZE and PTRACE_INTERRUPT
to guarantee that the target stops immediately.

This also has the advantage of never putting the process in group-stop
state, which means if we exit prematurely (e.g. if we crash during
stack unwinding), the target process will be resumed without any
intervention needed.

Bug: http://b/34472671
Test: while true; do debuggerd -b `pidof audioserver`; done
Change-Id: I7549f67489646cf300b8c9aa8c735e5e897806ef