[Thread] make Thread flags fixed_read_only am: 8fb3f07dfe am: 255c23a600

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/3243931

Change-Id: I52c93baf327e6917abfa0189a3a1c37b9de04647
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>