add a property for controlling perf_event_paranoid

This adds a system property for controlling unprivileged access to
perf_event_paranoid. It depends on adding kernel support for
perf_event_paranoid=3 based on grsecurity's PERF_HARDEN feature to
completely disable unprivileged access to perf. A minimal port of this
feature is used in the vanilla Debian kernel by default.

It hides the non-hardened value as an implementation detail, since while
it is currently 1, it will probably become 2 in the future.

Bug: 29054680

Change-Id: I6e3ae3cf18d8c76df94f879c34fb6fde519b89a9