Fix NPE in TransportBorker

[JulianKast]

Fixes #1881

This PR is [ready] for review.

Risk

This PR makes [no] API changes.

Testing Plan

• I have verified that I have not introduced new warnings in this PR (or explain why below)
• I have run the unit tests with this PR
• I have tested this PR against Core and verified behavior (if applicable, if not applicable, explain why below).
• I have tested Android

Unit Tests

n/a

Core Tests

Test 1: (This is from our test plan using test suite and hello_sdl)

1. This test needs two apps (app1 is custom RS, hello_sdl)
2. Use test suite customRS flavor as an app1, start the app, then go to menu and click on "Start Custom RS"
3. Use hello_sdl with multi_sec_off flavor for the second app but do the following changes:
   i. Change the UUID in MultiplexBluetoothTransport.SERVER_UUID to any other UUID
   ii. Modify the way MainActivity starts the SdlService to manually start the SdlService instead of calling queryForConnectedServices
4. Start hello_sdl and make sure it connects through the custom RS

Test 2: (This is the accidental test I ran when I found the NPE)

1. Run test 1 but have second app use multi_high_bandwidth flavor for hello_sdl
2. Observe that the custom RS is connected, but app hello_sdl does not connect, no NPE in logs

Core version / branch / commit hash / module tested against: Sync 3
HMI name / version / branch / commit hash / module tested against: Sync 3

Summary

This PR fixes an NPE which can happen if we don't close the routerServiceMessageEmitter before we set routerServiceMessenger to null in TransportBroker.shutDownRouterServiceMessenger

Bug Fixes

• Fix NPE in TransportBroker

CLA

• I have signed the CLA