Separate exported includes out of flags

Exported includes have been maintained along with other C/C++ flags.
This makes dependencies unclear, and users have had to parse flags to
get exported directories. This separates exported includes and
exported flags, thus making data more structured and explicit.

Bug: 132818174
Test: m
Change-Id: I5c25ac2224988c4a67e4db6fd6e4d39090b74041

cc/ndk_prebuilt.go

@@ -151,7 +151,7 @@ func (ndk *ndkPrebuiltStlLinker) link(ctx ModuleContext, flags Flags,
 		ctx.ModuleErrorf("NDK prebuilt libraries must have an ndk_lib prefixed name")
 	}
 
-	ndk.exportIncludes(ctx, "-isystem ")
+	ndk.exportIncludesAsSystem(ctx)
 
 	libName := strings.TrimPrefix(ctx.ModuleName(), "ndk_")
 	libExt := flags.Toolchain.ShlibSuffix()