Make VPN more testable and update NC during network change

This commit:
- Updates the NetworkCapabilities with new underlying network during
  IKE Session re-establishment
- Create wrapper classes for IkeSession and NetworkAgent
- Allow injecting executor and NetworkAgent with
  the Dependencies class

Bug: 192077544
Test: atest VpnTest(new tests)
Change-Id: Ic0f0c54967f5f898f43ed126f4ef410adb2c1969