mapclient: human-readable timestamp logging and dumpsys

Make logging of timestamps human-readable instead of `long`. Also add
recent messages (including timestamps) of `Inbox` and `Sent` folders to
dumpsys.

Bug: 321281779
Test: atest MapClientContentTest
Test: adb shell dumpsys bluetooth_manager
Flag: EXEMPT, change to logging only
Change-Id: Ica9f68c6081582c4c3be8ef4d84ad3c0bfb292f5