ANDROID: uid_sys_stats: Remove obsolete cputime type and helpers

Upstream commit 605dc2b3 ("tsacct: Convert obsolete cputime
type to nsecs") made cputime_t type obsolete, use u64 nanoseconds
directly instead.

cputime_to_jiffies() is obsolete too. See upstream commit
f22d6df0 ("sched/cputime: Remove jiffies based cputime") for
reference. Thought of using nsecs_to_jiffies() instead but I think
replacing jiffies_to_msecs(nsecs_to_jiffies()) with ktime_to_ms()
to convert nsecs to msecs directly make more sense.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>