Enable tracing on user builds

This change adds init.rc steps to:
 * allow kernel tracing to be enabled via adb
 * allow a limited set of kernel trace events to be enabled via adb
 * allow the kernel trace to be read via adb
 * allow all users to write to the kernel trace from userland

Bug: 6513400
Change-Id: Ic3c189b5697aa5edf88d2f507c932971bed6caff