Use the tab's contentDescription as the toolbar title (#3609)

Previous code had an IndexOutOfBoundsException where tab.position could be larger than the tabs array.

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

@@ -737,7 +737,7 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, HasAndroidInje
 
         onTabSelectedListener = object : OnTabSelectedListener {
             override fun onTabSelected(tab: TabLayout.Tab) {
-                binding.mainToolbar.title = tabs[tab.position].title(this@MainActivity)
+                binding.mainToolbar.title = tab.contentDescription
 
                 refreshComposeButtonState(tabAdapter, tab.position)
             }