Build system support to export include paths

Bug: 5573756
With this change, we can set LOCAL_EXPORT_C_INCLUDE_DIRS in the module
definition to export include dir paths.
Paths in LOCAL_EXPORT_C_INCLUDE_DIRS should be relative to the top dir
of the source tree.
If a library (shared or static) exports some include paths, any module
using it will import the include paths and add them to the compiler
command line.

Change-Id: I49aabc589d2cf214044d13ccd5532ef68209adf0