Add ENABLE_TRANSIENT_TASKBAR feature flag. am: efd41a717e

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/Launcher3/+/20159824

Change-Id: I08f4be61a2f26ad7d95affe56245806713393a5f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>