Fix media span and task list memory leaks

[khaykov]

Fix

This PR fixes memory leaks in media and task list spans.

Test

1. Toggle items in task list, make sure they work as expected.
2. Delete images, confirm that you see a "media deleted" snackbar.
3. Perform same actions after changing device orientation or bringing up to foreground from background.

[zwarm]

@khaykov - Tested and all works as expected. 👍

I have one question: Might it be safer to use
val drawable = contextRef.get()?.resources?.getDrawable(R.drawable.ic_checkbox, null)

instead of

val d: Drawable = contextRef.get()!!.resources.getDrawable(R.drawable.ic_checkbox, null)

I am approving the PR, but will leave the merge for you so you'll have a chance to review my question.

[khaykov]

@zwarm Good point! Switched to elvis operator 👍