Allow RNDIS and NCM to be enabled at the same time.

Currently, UsbManager#setCurrentFunctions takes a bitmask of
functions to enable, but will only allow callers to enable one
function at a time.

Allow it to enable both RNDIS and NCM together as well. This is
because from the user's point of view the two functions are the
same (i.e., USB tethering).

Android has historically used RNDIS for tethering. NCM performs
better, but it doesn't work on older OSes like Windows 8.1.

Also change the import order to keep the linter happy.

Bug: 172793258
Test: added unit test for UsbManager.areSettableFunctions
Change-Id: I51b6752781528b09f8929e1fb3b1fa7e4ac36aae