Merge "[User Switcher] Improve documentation" into tm-qpr-dev

packages/SystemUI/docs/user-file-manager.md

@@ -1,10 +1,30 @@
 # UserFileManager
 
-This class is used to generate file paths and SharedPreferences that is compatible for multiple
+This class is used to generate file paths and SharedPreferences that is compatible for specific OS
 users in SystemUI. Due to constraints in SystemUI, we can only read/write files as the system user.
 Therefore, for secondary users, we want to store secondary user specific files into the system user
 directory.
 
+
+## Usages
+
+Inject UserFileManager into your class.
+
+### fun getFile(fileName: String, userId: Int): File
+Add a file name and user id. You can retrieve the current user id from UserTracker. This will
+return a java.io File object that contains the file path to write/read to.
+
+i.e. `fileManager.getFile("example.xml", userTracker.userId)`
+
+### fun getSharedPreferences(fileName: String, mode: Int, userId: Int): SharedPreferences
+Add a file name, user id, and PreferencesMode. You can retrieve the current user id from
+UserTracker. This returns SharedPreferences object that is tied to the specific user. Note that if
+the SharedPreferences file does not exist, one will be created automatically. See
+[SharedPreferences documentation](https://developer.android.com/reference/android/content/Context#getSharedPreferences(java.lang.String,%20int))
+for more details.
+
+i.e. `fileManager.getSharedPreferences("prefs.xml", userTracker.userId, 0)`
+
 ## Handling User Removal
 
 This class will listen for Intent.ACTION_USER_REMOVED and remove directories that no longer