debuggerd_handler: raise ambient capset before execing.

Raise the ambient capability set to match CapEff so that crash_dump can
inherit all of the capabilities of the dumped process to be able to
ptrace. Note that selinux will prevent crash_dump from actually use
any of the capabilities.

Bug: http://b/34853272
Test: debuggerd -b `pidof system_server`
Test: debuggerd -b `pidof zygote`
Change-Id: I1fe69eff54c1c0a5b3ec63f6fa504b2681c47a88