Docs: VPNs under Android O should promote to the foreground

Add note for VPN developers that VPN apps started in the background must transition to the foreground in Android O to avoid the system stopping them.

Staged at: go/dac-stage/reference/android/net/VpnService.html

Test: make ds-docs
Bug: 38023983
Change-Id: I33c7ca1717c332ffab495eb51c4c6b9c5c304cef
1 file changed