SyncAdapterService doesn't cancel on request

[rfc2822]

SyncAdapterService should cancel onPerformSync (but not the actual synchronization) when the sync framework requests cancellation by calling onSyncCanceled.

We have a variable for that:

davx5-ose/app/src/main/kotlin/at/bitfire/davdroid/sync/SyncAdapterServices.kt

Lines 64 to 73 in 3776b50

	/**
	* Completable [Boolean], which will be set to
	*
	* - `true` when the related sync worker has finished
	* - `false` when the sync framework has requested cancellation.
	*
	* In any case, the sync framework shouldn't be blocked anymore as soon as a
	* value is available.
	*/
	private val finished = CompletableDeferred<Boolean>()

It's set to true when the sync framework requests cancellation, but it doesn't do anything. It should cause onPerformSync to cancel.

Probably we need to create a CoroutineScope (MainScope?) and cancel this scope instead, like in this example.

Also, this code comment seems wrong:

Instead we check periodically whether the sync has finished, putting the thread to sleep in between checks.

We don't check periodically, but use getWorkInfosForUniqueWorkFlow.