[TP] Refactor util function for instantiating preview fragment am: a2b70df7

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/ThemePicker/+/19970325



Change-Id: I6b3c7896c24b7ae7f9db3d357dd9138d6938d51d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>