Allow system_server to access /sys/kernel/debug/binder/state

For debugging watchdog issues in system_server, system_server
needs read access to binder-state file. Access to generic debugfs has
been removed for all processes except init, vendor_init & dumpstate.
This CL labels /sys/kernel/debug/binder/state file and allows
system_server, dumpstate, vendor_init & init, 'r_file_perms' access
to the same file.
The label and the associated access permissions only apply to
userdebug builds.

Change-Id: I159e39bcd05d699454797f8b1d1c17c810c99cb1