Google Find Hub as a second location network

[alex-so-3]

Adds Google Find Hub as a second location network next to Apple's.

The main use is tags that advertise on both networks, such as DIY trackers (e.g. Everytag-style firmware) that send Apple Find My and Google Find Hub frames at the same time. Such a tag is one device in the app, and the location reports from both networks are merged into its position and history. Tags that are only on Google work too.

Google reports come from a small self-hosted service, googlefind-service (Docker, built on GoogleFindMyTools). The app only talks to that service; nothing Google-specific is signed in on the phone. Without it configured, nothing changes.

What it adds:

• Settings → Extra location sources: the service URL, an API token, and optional HTTP Basic Auth for a reverse proxy.
• Add from Google Find Hub: adds a tracker of the Google account as a device.
• Link Google tracker: on an existing device, so reports from both networks land on the same device. The map card says which network a position came from.
• Google is asked for fresh locations only when the user refreshes a tag. A tracker with no reports yet is located once.
• Ringing Google trackers: the owner ring from the Find Hub (FMDN) spec, over the Beacon Actions characteristic. The ring key and current EIDs come from the service; the ring key can make the tracker ring but cannot decrypt its locations.
• Nearby for Google trackers: recognised by their EID in the 0xFEAA service data. Their battery flags are encrypted, so only the signal is shown.
• Add OpenHaystack key: imports a .keys file (Macless Haystack / OpenHaystack format), including files with several keys for tags that rotate through them. They become ordinary custom_rolling_key_accessory devices, fetched through the Apple account like any self-made tag.
• Ring failures on the map card are shown instead of being retried silently.

There is no Room schema change. The Google-specific data (the linked tracker id) is kept in SharedPreferences, because FindMy.py rewrites the accessory JSON and drops unknown keys. New strings are in all locales.

Depends on #242 and #241. This branch contains both commits: the ring code needs stopSound, and the map card and Nearby need the accessory JSON on the model. Once those are merged, the diff shrinks to the one feature commit.

Verified: on a phone with a Google account that has two off-the-shelf Find Hub trackers and one DIY tag on both networks:

• the Google trackers are added, located on refresh, ring from the map card and from the device menu, and show up in Nearby;
• the DIY tag linked to its Google tracker shows as one device with reports from both networks.

./gradlew testAll and add_strings.py --check pass. New JVM tests cover the FMDN ring command, report merging and .keys parsing.

Not verified:

• the instrumented suite: I did not add instrumented tests for the two new screens (settings and import dialogs);
• AMap with Google reports;
• the Find Hub ring on trackers other than the two cheap ones I have. Vendors may differ from the spec, so the app retries the few layouts seen in practice.