property_contexts

(Cherry picked from commit 38ac77e4)

This allows the shell user to control whether unprivileged access to
perf events is allowed.

To enable unprivileged access to perf:

    adb shell setprop security.perf_harden 0

To disable it again:

    adb shell setprop security.perf_harden 1

This allows Android to disable this kernel attack surface by default,
while still allowing profiling tools to work automatically. It can also
be manually toggled, but most developers won't ever need to do that if
tools end up incorporating this.

Bug: 29054680

Change-Id: Idcf6a2f6cbb35b405587deced7da1f6749b16a5f