Use global `jobject`s in `MonitorTest`. am: e7ea0461bb am: 4f599ba2d5

Original change: https://android-review.googlesource.com/c/platform/art/+/2491885

Change-Id: Idedb9c65d168c96f7ae24059d4f4d55dda8f4d01
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>