Check the system default theme on the fly

[TheHasagi]

We should check the system default theme on the fly, for now, it is complicated and it is a successful only for one change.

Preconditions:

Light or dark theme should be enabled on the device.

Steps to reproduce:

1. Open AdGuard - Settings - General - (theme) System default (check)
2. Go to system settings and change the theme to dark/light
3. Check AdGuard application

Expected result:

AdGuard should change the theme to the system default.

Actual result:

AdGuard did not change the theme automatically to the system default.