Add LOCAL_EXPORT_CFLAGS for Soong

This lets Soong pass -isystem or -I as necessary (or potentially, even
other cflags in the future).

This is not available for Android.mk use, nor exposed directly to
Android.bp users.

Test: m -j
Merged-In: Id37d4692d5fbddce467bd777903b20169f44dd6e
Change-Id: Id37d4692d5fbddce467bd777903b20169f44dd6e