crash_dump: read /proc/<pid>/maps before dropping capabilities.

Reading /proc/<pid>/maps does a ptrace_may_access check, which will
fail if we have fewer capabilities than the target, even if we've
already ptraced it.

Bug: http://b/35070339
Test: debuggerd -b `pidof zygote`
Change-Id: I984a061022bd945a7950b88f6d579e1bd735e893
(cherry picked from commit 347164cc)