[automerger skipped] Throw UnsupportedOperationException if startLegacyVpn is called am: 3eb2a54e31 am: 1695d3f5a1 -s ours am: 368cfbd99a -s ours am: c7987c33b5 -s ours

am skip reason: Merged-In I00f49555bbbc8ccf270056f0e4d1bfdd927ef26d with SHA-1 d4c4a2c95b is already in history

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1746333

Change-Id: I10a6159b2d5cfeeafacf14efec200339c26ee820