restrict access to timing information in /proc

These APIs expose sensitive information via timing side channels. This
leaves access via the adb shell intact along with the current uses by
dumpstate, init and system_server.

The /proc/interrupts and /proc/stat files were covered in this paper:

https://www.lightbluetouchpaper.org/2016/07/29/yet-another-android-side-channel/

The /proc/softirqs, /proc/timer_list and /proc/timer_stats files are
also relevant.

Access to /proc has been greatly restricted since then, with untrusted
apps no longer having direct access to these, but stricter restrictions
beyond that would be quite useful.

Change-Id: Ibed16674856569d26517e5729f0f194b830cfedd