Make the test code compatible with -Wthread-safety am: d4de1742ce am: 57c30bbddc

Original change: https://android-review.googlesource.com/c/platform/frameworks/minikin/+/2313179

Change-Id: I160d125d2b38fd31e2b4a662b9a784e8b426bae8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>