[Phone Hub] Log AppListUpdate and improve MessageReceiver coverage

AppListUpdate messages received were not being logged properly and were
throwing "UNKNOWN_MESSAGE". This change introduces it to help with
debugging/stabilization efforts. Additionally, this CL improves
MessageReceiver to go from ~70% coverage to 90%.

Test: manually verified on chrome://proximity-auth
Bug: b/258303790
Change-Id: Iccde730e49189c6dd440d683ec1858e4df4940d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4226721
Reviewed-by: Pu Shi <pushi@google.com>
Commit-Queue: Crisrael Lucero <crisrael@google.com>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Jon Mann <jonmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1102403}