Add active peer notification (#273)

* adding notification for active peers + tests

* adding a recently seen peer set to track if we've seen that peer before

* changing back to notificationManager naming

* fixing some weird formatting that occurred during merge conflict fix

app/src/main/java/com/bitchat/android/util/NotificationIntervalManager.kt

@@ -0,0 +1,13 @@
+package com.bitchat.android.util
+
+class NotificationIntervalManager {
+  private var _lastNetworkNotificationTime = 0L
+  val lastNetworkNotificationTime: Long
+    get() = _lastNetworkNotificationTime
+
+  val recentlySeenPeers: MutableSet<String> = mutableSetOf()
+
+  fun setLastNetworkNotificationTime(notificationTime: Long) {
+    _lastNetworkNotificationTime = notificationTime
+  }
+}