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

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

Change-Id: Ieb252b0dd5cc4281bf492716ec227d9fcd041101
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>