[automerger skipped] Add test for Vpn#prepare() am: a36535a2c8 -s ours

am skip reason: Merged-In Ibe0b245561b00442a6377fb849603f8b9026e9c4 with SHA-1 c01630e963 is already in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/16524179

Change-Id: I09cb8f0067ff43dba3fe12a93b53b8c66aa6c726