Provide debug hook for call blocking / E911.

Add a systems settings config, which is checked for the delay
threshold if a particular debug VERBOSE logging mode is used.

Usage:
adb shell setprop log.tag.DebugEmergencyCall VERBOSE
adb shell settings put system dialer_emergency_call_threshold_ms 60000

Bug: 25216954
Change-Id: Ia9b179e16819d3d8232cbedd49d5744d7b69a825