TrackList: Fix crashes in selection mode

[lmarz]

Fixes: #1711

The long click crash happened, because we didn't check whether we are already in selection mode.

The context menu item crash happened in setAllSelected, because item count != view holder count. That was a design error of me that the SparseBooleanArray uses the view holder positions, which change their contents, as the keys. So now I switched to using the trackIds, which are stored as Integers in the database, so we can easily convert them to long and back. This also simplifies the getSelectedIds method. The values from getSelectedPositions don't get used in the TrackListActivity, so we can just use null.

In regards to the recyclerview-selection library you mentioned, I deliberately decided against it, because

1. It's an extra library we would need to include.
2. The API, in my opinion, is awful. The method I used with the SparseBooleanArray is much more intuitive and easier to read to me.

I won't reimplement the selection mode using this library, but if someone gets along with the API and wants to implement this, feel free to :)

[dennisguse]

Works!
Thx.

@lmarz is the MarkerList also affected by this?

[lmarz]

Yes, thanks for reminding me

[dennisguse]

Fix is released as v4.9.1