Add flag to default enable pci tunnels

For testing and dogfooding before the UI portion is added, create a flag
that will default enable pci tunnels for USB4 and Thunderbolt.

Bug: 369430205
Test: m
Flag: com.android.server.usb.flags.default_allow_pci_tunnels
Change-Id: I1a6c44385d159a667ada77304cf0d45b01f079eb