check if Android sdk folder is writable

Fixes: QTCREATORBUG-21943
Change-Id: Ic20ed4c24c567b0b08269b7ad07014eb4248b214
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/fileutils.h

@@ -82,6 +82,7 @@ public:
 
     QString fileName(int pathComponents = 0) const;
     bool exists() const;
+    bool isWritablePath() const;
 
     FilePath parentDir() const;