make sure MainActivity doesn't hide other activities when launcher icon is clicked again (#4813)

closes #4789 this time for real, hopefully, maybe

also set `alwaysRetainTaskState` for good measure

> `android:alwaysRetainTaskState`
Whether the state of the task that the activity is in is always
maintained by the system. `"true"` if it is, and `"false"` if the system
can reset the task to its initial state in certain situations. The
default value is `"false"`. This attribute is meaningful only for the
root activity of a task. It's ignored for all other activities.

> Normally, the system clears a task, removing all activities from the
stack above the root activity, in certain situations when the user
re-selects that task from the home screen. Typically, this is done if
the user hasn't visited the task for a certain amount of time, such as
30 minutes.

> However, when this attribute is `"true"`, users always return to the
task in its last state, regardless of how they get there. This is useful
in an application like a web browser where there is a lot of state, such
as multiple open tabs, that users don't want to lose.


https://developer.android.com/guide/topics/manifest/activity-element#always

app/src/main/java/com/keylesspalace/tusky/MainActivity.kt

@@ -211,6 +211,12 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, MenuProvider {
         }
         super.onCreate(savedInstanceState)
 
+        // make sure MainActivity doesn't hide other activities when launcher icon is clicked again
+        if ((intent.flags and Intent.FLAG_ACTIVITY_BROUGHT_TO_FRONT) != 0) {
+            finish()
+            return
+        }
+
         // will be redirected to LoginActivity by BaseActivity
         activeAccount = accountManager.activeAccount ?: return