修复获取外部存储目录时的NPE

[iamwent]

Context.getExternalFilesDirs 这个方法 returns the absolute paths to application-specific directories. Some individual paths may be null if that shared storage is not currently available. The first path returned is the same as getExternalFilesDir(String). 也就是说 files[0].getAbsolutePath() 可能会产生 NPE

[jenly1314]

context.getExternalFilesDir是一个过时的方法，从API 19之后就不推荐使用了。