文件下载cordovaFileTransfer：cordova.file.documentsDirectory is null

在Android平台上使用：$cordovaFileTransfer进行文件下载时提示：cordova.file.documentsDirectory is null，查了以下文档参照：http://ngcordova.com/docs/plugins/file/，

documentsDirectory是IOS平台的用法，在Android平台上没有此方法所以获取到null,解决方法：

通过使用：$cordovaDevice.getPlatform()获取应用平台，针对不同平台进行文件下载处理：

IOS使用:cordova.file.documentsDirectory

Android使用:cordova.file.externalRootDirectory

iOS File System Layout

Device Path	cordova.file.*	iosExtraFileSystems	r/w?	persistent?	OS clears	sync	private
/var/mobile/Applications/<UUID>/	applicationStorageDirectory	-	r	N/A	N/A	N/A	Yes
appname.app/	applicationDirectory	bundle	r	N/A	N/A	N/A	Yes
www/	-	-	r	N/A	N/A	N/A	Yes
Documents/	documentsDirectory	documents	r/w	Yes	No	Yes	Yes
NoCloud/	-	documents-nosync	r/w	Yes	No	No	Yes
Library	-	library	r/w	Yes	No	Yes?	Yes
NoCloud/	dataDirectory	library-nosync	r/w	Yes	No	No	Yes
Cloud/	syncedDataDirectory	-	r/w	Yes	No	Yes	Yes
Caches/	cacheDirectory	cache	r/w	Yes*	Yes***	No	Yes
tmp/	tempDirectory	-	r/w	No**

Android File System Layout

Device Path	cordova.file.*	AndroidExtraFileSystems	r/w?	persistent?	OS clears	private
file:///android_asset/	applicationDirectory		r	N/A	N/A	Yes
/data/data/<app-id>/	applicationStorageDirectory	-	r/w	N/A	N/A	Yes
cache	cacheDirectory	cache	r/w	Yes	Yes*	Yes
files	dataDirectory	files	r/w	Yes	No	Yes
Documents		documents	r/w	Yes	No	Yes
<sdcard>/	externalRootDirectory	sdcard	r/w	Yes	No	No
Android/data/<app-id>/	externalApplicationStorageDirectory	-	r/w	Yes	No	No
cache	externalCacheDirectry	cache-external	r/w	Yes	No**	No
files	externalDataDirectory	files-external	r/w