Add an item that opens system notification settings

This item is displayed only on Oreo or higher and is hidden in previous
versions of Android.

app/ui/src/main/java/com/fsck/k9/ui/settings/account/NotificationsPreference.kt

@@ -0,0 +1,37 @@
+package com.fsck.k9.ui.settings.account
+
+import android.annotation.SuppressLint
+import android.content.Context
+import android.content.Intent
+import android.provider.Settings
+import android.support.v4.app.DialogFragment
+import android.support.v4.content.ContextCompat.startActivity
+import android.support.v4.content.res.TypedArrayUtils
+import android.support.v7.preference.Preference
+import android.util.AttributeSet
+import com.takisoft.fix.support.v7.preference.PreferenceFragmentCompat
+
+@SuppressLint("RestrictedApi")
+class NotificationsPreference
+@JvmOverloads
+constructor(
+        context: Context,
+        attrs: AttributeSet? = null,
+        defStyleAttr: Int = TypedArrayUtils.getAttr(context, android.support.v7.preference.R.attr.preferenceStyle,
+                android.R.attr.preferenceStyle),
+        defStyleRes: Int = 0
+) : Preference(context, attrs, defStyleAttr, defStyleRes) {
+
+    override fun onClick() {
+        val intent = Intent(Settings.ACTION_APP_NOTIFICATION_SETTINGS)
+        intent.putExtra(Settings.EXTRA_APP_PACKAGE, this.context.packageName)
+        startActivity(this.context, intent, null)
+    }
+
+    companion object {
+        init {
+            PreferenceFragmentCompat.registerPreferenceFragment(
+                    NotificationsPreference::class.java, DialogFragment::class.java)
+        }
+    }
+}

app/ui/src/main/res/values/strings.xml

@@ -558,6 +558,8 @@ Please submit bug reports, contribute new features and ask questions at
     <string name="account_settings_notification_opens_unread_summary">Searches for unread messages when Notification is opened</string>
     <string name="account_settings_mark_message_as_read_on_view_label">Mark as read when opened</string>
     <string name="account_settings_mark_message_as_read_on_view_summary">Mark a message as read when it is opened for viewing</string>
+    <string name="account_settings_notification_open_system_notifications_label">Notification settings</string>
+    <string name="account_settings_notification_open_system_notifications_summary">Open system notification settings</string>
 
     <string name="account_settings_show_pictures_label">Always show images</string>
     <string name="account_settings_show_pictures_never">No</string>

app/ui/src/main/res/xml/account_settings.xml

@@ -392,6 +392,11 @@
             android:summary="@string/account_settings_notification_opens_unread_summary"
             android:title="@string/account_settings_notification_opens_unread_label" />
 
+        <com.fsck.k9.ui.settings.account.NotificationsPreference
+            android:key="open_notification_settings"
+            android:summary="@string/account_settings_notification_open_system_notifications_summary"
+            android:title="@string/account_settings_notification_open_system_notifications_label" />
+
     </PreferenceScreen>
 
     <PreferenceScreen