CM11 Themes: Common overlay for sharing resources. [1/2]

This patch will allow themes to contain common resources
which can be shared by all overlays.

Common resources will reside in assets/overlays/common and
can be referenced by other overlays.

Common resources will be referenced using the common package
name, i.e. To reference the common drawable some_drawable for
com.example.theme the theme designer would use
"@*common:drawable/some_drawable"

Change-Id: I395b6bd5eae91b00265a8e9221856ef7bf5d115a
(cherry picked from commit 2bb685a05dfa5ddc70a52997a75feda5c58834d3)

Conflicts:
	core/java/android/content/res/AssetManager.java
	include/androidfw/AssetManager.h