Enable MTP or ADB when default functions are enabled

Clients (viz. Tethering) use UsbManager@setCurrentFunction(null, ..)
to make the device switch to default functions. While in OemMode,
this gets set to the last enabled functions list. Instead,
enable MTP or ADB depending on whether ADB is enabled or not.
This is the expected behavior for normal boot as well.

Bug: 37491031
Test: Verified that the device falls back to either mtp or adb while
in OemOverride mode.
Change-Id: I1c26d1d0ee3c015b597d40771cd765b783cd91bb