Stop depending on the ConnectivityService default request.

This avoids the need to expose ConnectivityService implementation
details as @SystemApi and the complexities that would be required
to make NetworkCapabilities parcelable in stable AIDL.

Bug: 112869080
Test: atest FrameworksNetTests NetworkStackTests
Change-Id: Icd5d25b9b8e9b7a7d1899b000c92dc4976c3b3d7