Start MtpDocumentsService as foreground service.

Prevously MtpDocumentsService was started as background service, then it
turns into a foreground service by calling Service#startForeground.

The workflow did not work until this, because now background activity
cannot launch a background service.

The CL starts using NotificationManager#startForegroundService to launch
MtpDocumentsService so that the service can be started as foreground
service directly.

Bug: 34468813
Test: MtpDocumentsProviderTests
Test: manual testing with connecting MTP device to Android
Change-Id: Ic35d3e92f234881846e5d82ed04d6681a83035f7