Show a warning for wrong system time during startup

Test instructions: Stop mailbox. change system date a few days into the past. start mailbox. expected behavior: clock error screen appears and after fixing system date, start continues. Addition: Try different time differences such as far in the past and future or just a few hours in the past and future.

Closes #138 (closed)

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

+/*
+ *     Briar Mailbox
+ *     Copyright (C) 2021-2022  The Briar Project
+ *
+ *     This program is free software: you can redistribute it and/or modify
+ *     it under the terms of the GNU Affero General Public License as
+ *     published by the Free Software Foundation, either version 3 of the
+ *     License, or (at your option) any later version.
+ *
+ *     This program is distributed in the hope that it will be useful,
+ *     but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *     GNU Affero General Public License for more details.
+ *
+ *     You should have received a copy of the GNU Affero General Public License
+ *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ */
+
+package org.briarproject.mailbox.android.ui
+
+import android.os.Bundle
+import android.view.LayoutInflater
+import android.view.View
+import android.view.ViewGroup
+import androidx.fragment.app.Fragment
+import androidx.fragment.app.activityViewModels
+import androidx.navigation.fragment.findNavController
+import dagger.hilt.android.AndroidEntryPoint
+import kotlinx.coroutines.flow.collect
+import org.briarproject.mailbox.R
+import org.briarproject.mailbox.android.StatusManager.ErrorClockSkew
+import org.briarproject.mailbox.android.StatusManager.MailboxAppState
+import org.briarproject.mailbox.android.ui.ClockSkewFragmentDirections.actionClockSkewFragmentToStartupFragment
+
+@AndroidEntryPoint
+class ClockSkewFragment : Fragment() {
+
+    private val viewModel: MailboxViewModel by activityViewModels()
+
+    override fun onCreateView(
+        inflater: LayoutInflater,
+        container: ViewGroup?,
+        savedInstanceState: Bundle?,
+    ): View? {
+        return inflater.inflate(R.layout.fragment_clock_skew, container, false)
+    }
+
+    override fun onViewCreated(v: View, savedInstanceState: Bundle?) {
+        launchAndRepeatWhileStarted {
+            viewModel.appState.collect { onSetupStateChanged(it) }
+        }
+    }
+
+    private fun onSetupStateChanged(state: MailboxAppState) {
+        if (state != ErrorClockSkew) {
+            findNavController().navigate(actionClockSkewFragmentToStartupFragment())
+        }
+    }
+
+}