Ensure audio session is default after call is terminated.

Previously, we were enabling PlayAndRecord when a call ended due to the
CallService observer's "updateVideoTrack" callbacks.

// FREEBIE

Signal/src/ViewControllers/AboutTableViewController.m

@@ -90,6 +90,13 @@
     NSString *_Nullable voipToken = [preferences getVoipToken];
     [debugSection addItem:[OWSTableItem labelItemWithText:[NSString stringWithFormat:@"Push Token: %@", pushToken ?: @"None" ]]];
     [debugSection addItem:[OWSTableItem labelItemWithText:[NSString stringWithFormat:@"VOIP Token: %@", voipToken ?: @"None" ]]];
+
+    // Strip prefix from category, otherwise it's too long to fit into cell on a small device.
+    NSString *audioCategory =
+        [AVAudioSession.sharedInstance.category stringByReplacingOccurrencesOfString:@"AVAudioSessionCategory"
+                                                                          withString:@""];
+    [debugSection
+        addItem:[OWSTableItem labelItemWithText:[NSString stringWithFormat:@"Audio Category: %@", audioCategory]]];
 #endif
 
     self.contents = contents;

Signal/src/call/CallAudioService.swift

@@ -159,6 +159,7 @@ protocol CallAudioServiceDelegate: class {
 
         super.init()
 
+        // This fails when someone toggles iOS Call Integration
         SwiftSingletons.register(self)
 
         // Configure audio session so we don't prompt user with Record permission until call is connected.
@@ -242,7 +243,8 @@ protocol CallAudioServiceDelegate: class {
     private func ensureProperAudioSession(call: SignalCall?) {
         AssertIsOnMainThread()
 
-        guard let call = call else {
+        guard let call = call, !call.isTerminated else {
+            // Revert to default audio
             setAudioSession(category: AVAudioSessionCategorySoloAmbient,
                             mode: AVAudioSessionModeDefault)
             return

Signal/src/call/CallService.swift

@@ -1507,6 +1507,9 @@ protocol CallServiceObserver: class {
         Logger.info("\(self.logTag) clearing pendingIceUpdateMessages")
         self.pendingIceUpdateMessages = []
         self.fulfillCallConnectedPromise = nil
+
+        // In case we're still waiting on this promise somewhere, we need to reject it to avoid a memory leak.
+        // There is no harm in rejecting a previously fulfilled promise.
         if let rejectCallConnectedPromise = self.rejectCallConnectedPromise {
             rejectCallConnectedPromise(CallError.obsoleteCall(description: "Terminating call"))
         }

Signal/src/call/SignalCall.swift

@@ -43,6 +43,15 @@ protocol CallObserver: class {
     var observers = [Weak<CallObserver>]()
     let remotePhoneNumber: String
 
+    var isTerminated: Bool {
+        switch state {
+        case .localFailure, .localHangup, .remoteHangup, .remoteBusy:
+            return true
+        case .idle, .dialing, .answering, .remoteRinging, .localRinging, .connected:
+            return false
+        }
+    }
+
     // Signal Service identifier for this Call. Used to coordinate the call across remote clients.
     let signalingId: UInt64