Background service for sending

[ShortDevelopment]

Issue

Currently, if a users closes the app or the screen goes dark during the transfer does not finish.

android/src/SendActivity.cs

Lines 382 to 387 in 4c74252

	public override void Finish()
	{
	base.Finish();
	_cdp?.Dispose();
	}

Solution

Move the cdp instance into a common background service that will be used for all transfers.

[sentry]

Sentry Issue: ANDROID-11

[dXrayb]

+1, if transfer a large file, you need to keep the screen on.