Tethering: add in-process vs out-of-process flag

This change results in the existence of either:
  /apex/com.android.tethering/etc/flag/out-of-process
  (Phones with mainline updatable Tethering in network_stack process)
or
  /apex/com.android.tethering/etc/flag/in-process
  (Android Go with InProcessTethering in system_server process)

These flags provide an easy way for the BpfLoader to
detect the required selinux context for /sys/fs/bpf/tethering
directory.

Bug: 190523685
Bug: 236925089
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I8e66806d81893885a5ebe8a6dd4194c5b9dae219