DO NOT MERGE: Support LOCAL_EXPORT_C_INCLUDE_DIRS for prebuilts too

Cherry picked from commit fe3f62d0
in master.  Since this is already in master, and we're upstream,
set to DO NOT MERGE.

Change-Id: I09e8e849f642792e3a74e2564820ab9bbf1eb28c
Signed-off-by: Mike J. Chen <mjchen@google.com>