Fix service start crashes (streaming and USB)

[zeevy]

Fixes three crashes from the Play Console.

#186 - ForegroundServiceDidNotStartInTimeException after Continue on the start streaming dialog

• FileStreamerIntentService is started with startForegroundService() but called startForeground() only deep in onHandleIntent, on the worker thread. With no G-code file it returned early without it, the service stopped, and Android killed the app.
• Now startForeground() runs first in onStartCommand, on every start, with the same notification (checking or streaming, file name). The two later calls and their always-true SDK_INT > N_MR1 checks (minSdk 26) are gone.

RemoteServiceException on Android 10 (no message in the trace)

• Most likely the Android 11-and-older form of the same "did not call startForeground" crash. Its two known sources are the streaming start (this PR) and the USB screen's old startForegroundService() in onStart (already fixed in v2.61, ed7501f). No extra code.

#189 - BackgroundServiceStartNotAllowedException in UsbConnectionActivity.onStart

• startService() there throws while Android counts the app as in the background. The bind in onCreate already creates and keeps the service alive (onStartCommand does nothing), so the refusal is now caught and logged.

Checked on the emulator (API 37)

• No machine can connect to the emulator, so throwaway builds (not committed) skipped the fragment's file and connection checks, and made the USB startService() call 8 s after onStop with am make-uid-idle in between.
• Old code: both crashes reproduced with the same exceptions.
• New code: Continue with no file shows "No gcode file selected", no crash, no notification left. The refused USB start logs a warning, no crash.
• Real build: normal launch starts the USB service as before (startRequested=true).
• ./gradlew build passes.