stats: Add MainPrintUiLaunched event

We want to log when the main print UI is launched.

Log the main print UI and its found printers/printservices.

Bug: 390478410
Test: atest PrintSpoolerRobolectricTests
Test: m PrintSpooler
Test: validate logged events via adb and webui
Flag: com.android.printspooler.flags.printing_telemetry
Change-Id: I7dc3304ae255166882e794d228c09b8e6e62365b