Add flag to skip usbnet disable

Some platforms need the service running
till the SMBIOS table transfers are complete.
They can use this flag and later disable
the service separately.

Tested:
Tested with the empty flag and verified
that the service is disabled.
https://paste.googleplex.com/5759611132051456

Tested with the non-empty flag and
verified that the service keeps running.
https://paste.googleplex.com/4673695156142080

Google-Bug-Id: 487678070
Platforms-Affected: platform31
Fusion-Link: fusion2, N/A
Change-Id: Iaea4c29d8ee2b2f5ba181af3f0b351e7553eea30
(cherry picked from commit d92fdd19d96e68cab2224e18e9ea7debe9a6b348)
Signed-off-by: Swati Wagh <swatiwagh@google.com>