Add SetFlagRule to WallpaperPreviewActivityTest (2/3)

This fixes test failure due to multi crop flag not being enabled. Multi
crop flags on the wallpaper and system side should always be enabled
when running WallpaperPreviewActivityTest.

Dependencies for SetFlagsRule are added in Android.bp files as well as
in WPPG's build.gradle.kts to enable test run on soong & gradle.

Flag: ACONFIG com.android.wallpaper.multi_crop_preview_ui_flag DEVELOPMENT
Bug: 322500606
Test: atest WallpaperPicker2Tests:WallpaperPreviewActivityTest
Test: gradle build WallpaperPreviewActivityTest
Change-Id: Icd8951bfc62eec63c5b9ba50f139b834c36b30d9