ANDROID: cpufreq_stats: Fix task time in state locking

The task->time_in_state pointer is written to at task creation
and exiting, protection is needed for concurrent reads e.g. during
sysfs accesses.  Added spin lock such that the task's time_in_state
pointer used is set to either allocated memory or null.

Bug: 38463235
Test: Torture concurrent sysfs reads with short lived tasks
Signed-off-by: Andres Oportus <andresoportus@google.com>
Change-Id: Iaa6402bf50a33489506f2170e4dfabe535d79e15