Allow debug-overrides to be specified in an extra resource

An application can specify its debug-overrides in an extra resource with
the same name suffixed with "_debug" (e.g. res/xml/security_config.xml and
res/xml/security_config_debug.xml).

By specifying the debug-overrides in an extra file release builds can
strip out the file (and any certificate resources that the
debug-overrides depend on) to prevent including testing configuration
information in the release build of an application.

Bug: 27418003
Change-Id: Ibfebc376360ca474fc0f9f2fd565faa0cffd9549