Fix bug: Grayscale feature does not turn off in allowlisted apps

[LucaMertens]

Hey, first of all: thank you for your work on the app! It's a great idea and I've been using it for a while now.

For the last couple of releases (after upgrading from a previous release), my phone would stay in grayscale mode, even when switching to an allowlisted app. This only happens when the grayscale feature is setup while grayscale is already active.
I believe #143 could also be explained by this.

Steps to reproduce

• Turn on grayscale mode
• Newly install or upgrade DetoxDroid
• Enable the grayscale feature
• The app saves the current grayscale state as the default (in the form of DISPLAY_DALTONIZER_ENABLED = 1 and DISPLAY_DALTONIZER = 0)
• When switching to an allowlisted app, DetoxDroid restores these defaults. The device remains stuck in grayscale mode.

The fix

Have the app check the current grayscale state at the start of the service and update the internal tracking variable. This prevents saving the grayscale settings as a default.

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I tried to keep the change as minimally invasive as possible. I'd be happy to see it merged :)