Remove the usage of hidden API - setIsAvailable() from Vpn

In fact, NetworkAgent#getLegacyNetworkInfo() will call this API,
so VPN doesn't need to call it.
Remove the redundant behavior and the usage of hidden API.

Bug: 182963397
Test: m
Change-Id: I3b88f22fffde3dd4000d28342ed07209c7121a37
1 file changed