fix: promote StreamingService to foreground immediately on Android 8+

[7MS8]

Split out of #406 per reviewer request (separate unrelated fixes into their own PRs).

Bug

On GrapheneOS (and strictly enforced Android 12+), streaming/opening a file could silently fail. StreamingService extended IntentService, which runs startForeground() on a background worker thread with non-deterministic scheduling delay. If that delay exceeds the ~5s the platform allows before killing the process, GrapheneOS kills the service before it ever reaches the foreground.

Fix

Convert StreamingService from the deprecated IntentService to a plain Service:

• Call startForeground() in onStartCommand() on the main thread, guaranteeing foreground promotion before any background-process limits can apply.
• Run the blocking waitFor() in a dedicated thread instead.
• Return START_NOT_STICKY (user-initiated serve, no auto-restart).
• Add tryStartForegroundService() to ActivityHelper and use it for all StreamingService start calls.

Reproduction

• Device: Pixel 7a, GrapheneOS.
• App version: build from master prior to this fix (versionCode 410/417).
• Steps: open the file browser on any remote, tap a file to stream/preview it.
• Observed: streaming silently fails / file never opens; GrapheneOS kills the service before foreground promotion completes.
• After this fix: file opens/streams correctly, verified on the same device.

Test plan

• Verified on a physical Pixel 7a running GrapheneOS: streaming a remote file now works reliably.
• Local build compiles (./gradlew :app:compileOssDebugSources).