Change exportedDirs and exportedSystemDirs from []string to android.Paths

exportedDirs and exportedSystemDirs are now changed to android.Paths so
that we can later manipulate the paths via Rel(), etc.

Test: m

Change-Id: I6fb02ea4983bcebac351bc284f75b44885379e8f
diff --git a/cc/llndk_library.go b/cc/llndk_library.go
index 9cbe800..16e089e 100644
--- a/cc/llndk_library.go
+++ b/cc/llndk_library.go
@@ -146,9 +146,9 @@
 		}
 
 		if Bool(stub.Properties.Export_headers_as_system) {
-			stub.reexportSystemDirs(genHeaderOutDir.String())
+			stub.reexportSystemDirs(genHeaderOutDir)
 		} else {
-			stub.reexportDirs(genHeaderOutDir.String())
+			stub.reexportDirs(genHeaderOutDir)
 		}
 
 		stub.reexportDeps(timestampFiles...)