fix(android): target the selected device in adb reads; don't strand nav mode

Review fixes:
- ForegroundPackage/FocusedWindowPackage now take a serial and pass -s, so the
  foreground/scope guard works when several devices are attached (the --device
  path). Previously they ran bare `adb shell`, which errors with multiple
  devices, silently disabling app-scope enforcement. The sidecar client passes
  its serial through.
- Extract an adbArgs helper and route every adb call through it, removing four
  duplicated serial-arg builders.
- ForceThreeButtonNav now decides what to restore before changing anything: if
  the current mode is unknown or already 3-button it leaves nav untouched,
  instead of switching and then stranding the device in 3-button. Logic split
  into the pure navModeToRestore, now unit tested.

internal/driver/sidecar/client.go

@@ -54,7 +54,7 @@ func (c *Client) ForegroundApp(ctx context.Context) (string, error) {
 	if c.platform != "android" {
 		return "", nil
 	}
-	return android.ForegroundPackage(ctx)
+	return android.ForegroundPackage(ctx, c.serial)
 }
 
 // FocusedWindowApp reports the package owning the focused window. Only Android
@@ -64,7 +64,7 @@ func (c *Client) FocusedWindowApp(ctx context.Context) (string, error) {
 	if c.platform != "android" {
 		return "", nil
 	}
-	return android.FocusedWindowPackage(ctx)
+	return android.FocusedWindowPackage(ctx, c.serial)
 }
 
 // Dial connects to the sidecar gRPC server at the given address.