Add support for debug-overrides configuration

Debug overrides are only used if the application is debuggable in
order to help local debugging and development by trusting additional
CAs. In a non-debuggable version of the application the debug-overrides
are ignored.

Trust anchors in the debug override configuration have two key
differences from those in base-config and domain-config:
1) trust anchors in the debug-overrides are trusted for all connections
in addition to any trust anchors included in the relevant base/domain
configs.
2) By default trust anchors in the debug config override pins, as their
purpose is for connecting to non-standard servers for debugging and
testing and those servers should not be pinned in the production
configuration.

Change-Id: I15ee98eae182be0ffaa49b06bc5e1c6c3d22baee