Allow multiple directories in LOCAL_RESOURCE_DIR

Before, LOCAL_RESOURCE_DIR got messed up when the build system
tried to find the whole LOCAL_RESOURCE_DIR string in each
overlay directory.

This change splits LOCAL_RESOURCE_DIR before appending it to
the overlay directory paths.

I've verified that this now works:

LOCAL_RESOURCE_DIR := $(LOCAL_PATH)/overlay $(LOCAL_PATH)/res

Change-Id: Iea47f94a14720af0ca4b422aec30acf4acbe13cf