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

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

Change-Id: I4782c3bfbea0dd2c0b47820bf09bc3cc14d9649b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>