stats: Add PrinterDiscovery event

We want to log when new printers are added, what service discovered
them, and what their capabilities are.

Log the above information when adding new printers to the
PrintActivity.

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: Ib5c47402d8d553b35737a74928808f35645a3c2f