Validate OEM_PAID networks

- Validation is required for private DNS in strict mode. Validate
  internet networks with OEM_PAID capability as well.
- Disable sign-in notification on all restricted networks, not
  just OEM_PAID restricted networks. This includes, for example,
  carrier wifi networks that back VCN networks.
- Update unit tests.
- Fix some lint errors.

Test: Builds, Boots
Test: atest NetworkStackTests:com.android.server.connectivity.NetworkMonitorTest
Bug: 187156341
Bug: 181281776
Original-Change: https://android-review.googlesource.com/1688270
Merged-In: I4def3a0fc49a9995a4556ec580fc730baf826a1b
Change-Id: I4def3a0fc49a9995a4556ec580fc730baf826a1b