Guard update timestamp logic with lock.

Guard the logic to update property value timestamp with lock so that
they will not overlap. Otherwise, it is possible that another op will
update the timestamp to a newer value, which causes this op to fail
the timestamp freshness check.

Test: atest --iterations 10 FakeVehicleHardwareTest#testSubscribe_enableVUR
Bug: 308552957
Change-Id: I8f7a6356ba65e4470e851e59c20bb1f678c2d467