JNI: Faster jobject decoding by avoiding checks. am: 17491ac89f am: c6a6133098

Original change: https://android-review.googlesource.com/c/platform/art/+/1515333

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I03b76b9408532f9aaeca7892e29a4b936a009d4f