[Cleanup]: Resolve Android Studio Warnings (#13282)

[Siddheshjondhale]

Purpose / Description

Reducing Android Studio warnings by cleaning up unused code and fixing initialization.

Fixes

• Contributes towards [Cleanup]: Fix Android Studio Warnings #13282 [Cleanup]: Fix Android Studio Warnings

Approach

• Made the unmountReceiver property private in AppCompatPreferenceActivity.kt.
• Removed the unused import (import androidx.activity.addCallback) in CardTemplateBrowserAppearanceEditor.kt.
• Removed the unused property editedModelFileName in CardTemplateNotetype.kt.
• Fixed the unresolved reference to BuildConfig in CardTemplateNotetype.kt.
• Made the legacyAnkiDroidDirectory property private in CollectionHelper.kt.
• Removed the unused property fieldsAsBundleForPreview in NoteEditor.kt.
• Fixed the initialization of activity in RtlCompliantActionProvider.kt by assigning it immediately during declaration, rather than inside the init block. This ensures proper initialization and avoids potential issues.

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have performed a self-review of your own code

[welcome]

First PR! 🚀 We sincerely appreciate that you have taken the time to propose a change to AnkiDroid! Please have patience with us as we are all volunteers - we will get to this as soon as possible.