Add convenience helper `ArtMethod::NewObject()`. am: b9c73f8b8d am: 372aec7388 am: 259c8c8728

Original change: https://android-review.googlesource.com/c/platform/art/+/2314658

Change-Id: I4a6d10dffb1dcd71e1da74c6586d9fa7b1149912
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>