debuggerd_handler: set PR_SET_PTRACER before running crash_dump.

Set and restore PR_SET_PTRACER when performing a dump, so that when
Android is running on a kernel that has the Yama LSM enabled (and the
value of ptrace_scope is > 0), crash_dump can attach to processes and
print nice, symbolized stack traces.

Bug: 70992745
Test: kill -6 `pidof surfaceflinger` && logcat -d -b crash
      # in both sailfish and Chrome OS

Change-Id: If4646442c6000fdcc69cf4ab95fdc71ae74baaaf