expose control over unpriv perf access to shell

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

property_contexts

@@ -33,6 +33,7 @@ debug.                  u:object_r:debug_prop:s0
 debug.db.               u:object_r:debuggerd_prop:s0
 dumpstate.              u:object_r:dumpstate_prop:s0
 log.                    u:object_r:shell_prop:s0
+security.perf_harden    u:object_r:shell_prop:s0
 service.adb.root        u:object_r:shell_prop:s0
 service.adb.tcp.port    u:object_r:shell_prop:s0