Use "factory" method for creating db migrations

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What is it?

• Codebase improvement (dev facing)

Description of the changes in your PR

• Use "factory" method for creating db migrations

git diff is ugly but only real change is from

    val MIGRATION_XXX = object : Migration(DB_VER_X, DB_VER_X) {
        override fun migrate(db: SupportSQLiteDatabase) {

to

    val MIGRATION_XXX = Migration(DB_VER_X, DB_VER_X) { db ->

+ subsequent remove of one indent level

APK testing

The APK can be found by going to the "Checks" tab below the title. On the left pane, click on "CI", scroll down to "artifacts" and click "app" to download the zip file which contains the debug APK of this PR. You can find more info and a video demonstration on this wiki page.

Due diligence

• I read the contribution guidelines.
• The proposed changes follow the AI policy.
• [*] I tested the changes using an emulator or a physical device.

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When you will be reviewing it select "hide whitespace" in diff settings of github