Support parsing VcnGatewayConnectionConfig without UnderlyingNetworkPriorities

This commit fixes the NullPointerException issue in converting a
PersistableBundle without UnderlyingNetworkPriorities to a
VcnGatewayConnectionConfig, by adding a null check and building
the VcnGatewayConnectionConfig with the default
UnderlyingNetworkPriority list.

Bug: 209142575
Test: atest FrameworksVcnTests (new tests), CtsVcnTestCases
Change-Id: I888d2994c86ba250c3fc9ee1851dccb5e6ef3e77