Remove SanitizedHostnameMigration once device_id-keyed sync lands (follow-up to #273)

[TimeToBuildBob]

Background

#273 fixed the immediate hostname-fork issue by adding a one-shot migration (SanitizedHostnameMigration.kt, ~356 lines) that rewrites bucket hostname rows to the sanitized form before the datastore opens.

The root cause — sync identity is hostname-keyed, so any future sanitizer/casing change will fork again — is tracked in ActivityWatch/activitywatch#302 (device_id-keyed sync direction) and ActivityWatch/aw-server-rust#683 (aw-sync data-loss from duplicate folders).

Remaining work

When ActivityWatch/activitywatch#302 lands and aw-server-rust implements device_id-keyed sync, the migration machinery from #273 becomes obsolete and should be removed:

• mobile/src/main/java/net/activitywatch/android/SanitizedHostnameMigration.kt (356 lines)
• mobile/src/test/java/net/activitywatch/android/SanitizedHostnameMigrationTest.kt (327 lines)
• Remove calls to SanitizedHostnameMigration in BackgroundService.kt and SyncInterface.kt
• Remove HOSTNAME_MIGRATION_DONE preference key from AWPreferences.kt

Done when

• SanitizedHostnameMigration.kt and its test deleted
• All callsites removed
• CI green

Depends on

• Change how hostname/devices work activitywatch#302 — device_id-keyed sync direction (upstream gate)
• aw-sync: duplicate folders for one device_id silently truncate history on pull aw-server-rust#683 — aw-sync duplicate folder data-loss (upstream gate)

Closes via: #272