build: Introduce project pathmap and use it for qcom variants

The project pathmap tracks the path to the top level of a project.  The
following functions are provided:

 * project-set-path adds an entry.
 * project-set-path-variant adds a "variant" entry.
 * project-path-for retrieves an entry.

To use as a guard in Android.mk:

  ifeq ($(call my-dir),$(call project-path-for,projectname))

To use for include paths in Android.mk:

  LOCAL_C_INCLUDES += $(call project-path-for,projectname)/...

Set project pathmap for qcom project variants.

Change-Id: I8dceca72a1ba80fc7b1830c5ab285d444f530457