External storage exists but not writable (#2641)

* Some phones (like Huawei P9) are detecting external storage as available, but it's not writable.

library/src/main/java/com/bumptech/glide/load/engine/cache/ExternalPreferredCacheDiskCacheFactory.java

@@ -52,7 +52,7 @@ public File getCacheDirectory() {
         File cacheDirectory = context.getExternalCacheDir();
 
         // Shared storage is not available.
-        if (cacheDirectory == null) {
+        if ((cacheDirectory == null) || (!cacheDirectory.canWrite())) {
           return internalCacheDirectory;
         }
         if (diskCacheName != null) {