Add separate user consent for Platform VPNs

This change adds a new VPN user consent flow (using the same text) for
granting the lesser OP_ACTIVATE_PLATFORM_VPN. A new
PlatformVpnConfirmDialog is created as a subclass to preserve all logic,
but ensure the right appop is granted for the relevant dialog.

Intent extras were considered, but are inherently unsafe, since the
caller may add any extras that they would want.

Bug: 144246835
Test: FrameworksNetTests passing
Change-Id: Ia6f36207d43c3748f938430c2780dcf29e5623f3