Make VpnService synchronous API.

This eases VpnSettings on dealing with multiple-activity-instance problem
(i.e., SettingsActivity and VpnSettingsActivity).

+ Most of the code is moved from the VpnServices package to vpn/java/.
+ VpnManager and VpnServiceBinder are revised to provide synchronous API.
+ Add a new method isIdle() to IVpnService.aidl.

Related bug: 3293236 (need to deal with multiple-activity-instance problem)

Change-Id: I03afa3b3af85d7b4ef800683cd075c356a9266c4