feat(Android): add notification action buttons

[Dre-Van-den-Hooff-Codifly]

Adds an optional actions option that renders buttons on the foreground service notification, and emits notificationAction when one is pressed.

Closes #60.

Usage

await BackgroundService.start(task, {
    ...options,
    actions: [
        { id: 'pause', title: 'Pause' },
        { id: 'cancel', title: 'Cancel' },
    ],
});

BackgroundService.on('notificationAction', ({ id }) => {
    if (id === 'pause') pauseMyTask();
});

Passing a new actions array to updateNotification() swaps the buttons, so a
task can show pause while running and resume while paused.

Notes on the implementation

• The receiver is registered at runtime, not in the manifest. The
  notification only exists while the service — and therefore the process — is
  alive, so there is never a press with no receiver to take it. It also avoids
  an exported manifest receiver. Unregistered in invalidate() so a JS reload
  doesn't leak one.
• Each button gets its own PendingIntent request code. Sharing one makes
  every button deliver whichever intent was registered first.
• _normalizeOptions passes actions through. It runs on every
  updateNotification() too, so without this the buttons would disappear on the
  first progress update. Covered by a regression test.
• notificationAction is re-emitted through the existing EventEmitter, so
  it's consumed exactly like expiration.
• actions is optional and Android-only; no behaviour changes when it's absent,
  and no iOS changes.

Testing

• yarn ci passes: 15 tests, 100% coverage on index.js. Four new cases —
  actions passed through on start, kept across updateNotification(),
  undefined when not supplied, and the event re-emission.
• The Java isn't compiled by CI, so I verified it separately: it compiles
  against RN 0.85 / AGP, and the full flow (buttons rendering, presses driving
  pause/resume/cancel through the foreground service) is running in a shipped
  release build on a physical device.