crash_dump: fetch process/thread names before dropping privileges.

Processes that don't have dumpable set to 1 cannot have their
process/thread names read by processes that don't have all of their
capabilities. Fetch these names in crash_dump before dropping
privileges.

Bug: http://b/36237221
Test: debuggerd_test
Test: debuggerd -b `pidof android.hardware.bluetooth@1.0-service`
Change-Id: I174769e7b3c1ea9f11f9c8cbdff83028a4225783