Note should default to Preview if the mode was the active tab when exiting the note

In #764, we added the option to automatically detect the last-viewed tab so notes would open in the Edit tab by default unless the note is exited while the Preview tab is shown.

This is not happening anymore.

### Expected
1. To open a note, click on Preview, leave the note, and then go back to it and still see the Preview tab open.
2. To open a note, click on Preview, leave the note, choose _another_ MD note, and also see the Preview by default.

### Observed
The note always return to Edit mode.

### Reproduced
1. Open an MD note
2. Click on Preview
3. Leave the note
4. Go back to the note

Device Make: Motorola
Device Model: E6
Android Version: 9.0.0
Simplenote Version: 2.15.0

User report: 3750787-zen

Start by reproducing the reported flow in the Android app: open an MD note, switch to Preview, leave it, and reopen it or choose another MD note. Trace where the active tab is saved and restored, then verify that Preview remains selected after both expected flows and that existing Edit behavior is unchanged.

Written by the indexing model from the issue text.