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/kernel_headers.go

@@ -25,9 +25,7 @@ type kernelHeadersDecorator struct {
 func (stub *kernelHeadersDecorator) link(ctx ModuleContext, flags Flags, deps PathDeps, objs Objects) android.Path {
 	if ctx.Device() {
 		f := &stub.libraryDecorator.flagExporter
-		for _, dir := range ctx.DeviceConfig().DeviceKernelHeaderDirs() {
-			f.flags = append(f.flags, "-isystem "+dir)
-		}
+		f.reexportSystemDirs(ctx.DeviceConfig().DeviceKernelHeaderDirs()...)
 	}
 	return stub.libraryDecorator.linkStatic(ctx, flags, deps, objs)
 }