Add cancel button in STARTING state

mailbox-android/src/main/java/org/briarproject/mailbox/android/ui/StartupFragment.kt

@@ -23,6 +23,7 @@ import android.os.Bundle
 import android.view.LayoutInflater
 import android.view.View
 import android.view.ViewGroup
+import android.widget.Button
 import android.widget.TextView
 import androidx.fragment.app.Fragment
 import androidx.fragment.app.activityViewModels
@@ -53,6 +54,11 @@ class StartupFragment : Fragment() {
         }
 
         viewModel.startLifecycle()
+
+        v.findViewById<Button>(R.id.button).setOnClickListener {
+            viewModel.stopLifecycle()
+            requireActivity().finishAffinity()
+        }
     }
 
     private fun onAppStateChanged(state: MailboxAppState) {

mailbox-android/src/main/res/layout/fragment_startup.xml

@@ -46,4 +46,16 @@
         app:layout_constraintStart_toStartOf="parent"
         app:layout_constraintTop_toBottomOf="@id/statusHeadline" />
 
+    <Button
+        android:id="@+id/button"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_margin="16dp"
+        android:text="@string/cancel"
+        app:layout_constraintBottom_toBottomOf="parent"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="parent"
+        app:layout_constraintTop_toBottomOf="@+id/statusDetail"
+        app:layout_constraintVertical_bias="1.0" />
+
 </androidx.constraintlayout.widget.ConstraintLayout>