tethering apex: add ethtool binary

Afterwards:
  vsoc_x86:/ # ls -lZ /apex/com.android.tethering/bin/ethtool
  -rwxr-xr-x 1 root shell u:object_r:system_file:s0  96204 1969-12-31 16:00 /apex/com.android.tethering/bin/ethtool

  vsoc_x86:/ # ldd /apex/com.android.tethering/bin/ethtool
  linux-gate.so.1 => [vdso] (0xedc4d000)
  libc++.so => /apex/com.android.tethering/lib/libc++.so (0xed309000)
  libc.so => /apex/com.android.runtime/lib/bionic/libc.so (0xed3db000)
  libdl.so => /apex/com.android.runtime/lib/bionic/libdl.so (0xed2c3000)
  libm.so => /apex/com.android.runtime/lib/bionic/libm.so (0xed286000)

Apex-Size-Increase: (verify ~100kB)
Aosp-First: it is
Previous-Platform-Support: been debug only for a while
Test-Info: builds, boots, binary works
Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I1f7d12f2df2cf26476634e7eb84ca8f7622905f4