refactor: migrate away from deprecated onBackPressed()

Author: naveensingh

app/src/main/kotlin/org/fossify/paint/activities/MainActivity.kt

@@ -238,7 +238,7 @@ class MainActivity : SimpleActivity(), CanvasListener {
         }
     }
 
-    override fun onBackPressed() {
+    override fun onBackPressedCompat(): Boolean {
         val hasUnsavedChanges = savedPathsHash != binding.myCanvas.getDrawingHashCode()
         if (hasUnsavedChanges && System.currentTimeMillis() - lastSavePromptTS > SAVE_DISCARD_PROMPT_INTERVAL) {
             lastSavePromptTS = System.currentTimeMillis()
@@ -252,12 +252,12 @@ class MainActivity : SimpleActivity(), CanvasListener {
                 if (it) {
                     trySaveImage()
                 } else {
-                    super.onBackPressed()
+                    performDefaultBack()
                 }
             }
-        } else {
-            super.onBackPressed()
+            return true
         }
+        return false
     }
 
     private fun launchSettings() {