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/vndk.go b/cc/vndk.go
index 14bbf11..1c71a49 100644
--- a/cc/vndk.go
+++ b/cc/vndk.go
@@ -463,8 +463,8 @@
 				RelativeInstallPath string   `json:",omitempty"`
 			}{}
 			prop.ExportedFlags = l.exportedFlags()
-			prop.ExportedDirs = l.exportedDirs()
-			prop.ExportedSystemDirs = l.exportedSystemDirs()
+			prop.ExportedDirs = l.exportedDirs().Strings()
+			prop.ExportedSystemDirs = l.exportedSystemDirs().Strings()
 			prop.RelativeInstallPath = m.RelativeInstallPath()
 
 			propOut := libOut + ".json"
@@ -523,7 +523,7 @@
 
 		generatedHeaders = append(generatedHeaders, l.exportedDeps()...)
 		for _, dir := range append(l.exportedDirs(), l.exportedSystemDirs()...) {
-			includeDirs[dir] = true
+			includeDirs[dir.String()] = true
 		}
 	})