Opt-out for always-on VPN: rename API.

Rename the opt-out flag in AndroidManifest to
SERVICE_META_DATA_SUPPORTS_ALWAYS_ON
as directed by the API Council.

Bug: 64331776
Bug: 36650087
Test: runtest --path java/com/android/server/connectivity/VpnTest.java
Change-Id: I24326fad7a89083a2409134640bda81ee0359d08
Merged-In: I24326fad7a89083a2409134640bda81ee0359d08
(cherry picked from commit c57a01c1)