Opt-out for always-on VPN

Always-on VPN is a feature introduced in N. Since then, all VPN apps
targeting N+ are assumed to support the feature, and the user or the DPC
can turn on / off always-on for any such VPN app. However, a few VPN
apps are not designed to support the always-on feature. Enabling
always-on for these apps will result in undefined behavior and confusing
"Always-on VPN disconnected" notification.

This feature provides a new manifest meta-data field through which a VPN
app can opt out of the always-on feature explicitly. This will stop the
always-on feature from being enabled for the app, both by the user and
by the DPC, and will clear its existing always-on state.

A @hide API is provided to check whether an app supports always-on VPN.
Documentation is updated to reflect the behavior change.

Bug: 36650087
Test: runtest --path java/com/android/server/connectivity/VpnTest.java
Test: cts-tradefed run cts --module CtsDevicePolicyManagerTestCases --test 'com.android.cts.devicepolicy.MixedDeviceOwnerTest#testAlwaysOnVpnUnsupportedPackage'
Test: cts-tradefed run cts --module CtsDevicePolicyManagerTestCases --test 'com.android.cts.devicepolicy.MixedDeviceOwnerTest#testAlwaysOnVpnUnsupportedPackageReplaced'
Test: cts-tradefed run cts --module CtsDevicePolicyManagerTestCases --test 'com.android.cts.devicepolicy.MixedProfileOwnerTest#testAlwaysOnVpnUnsupportedPackage'
Test: cts-tradefed run cts --module CtsDevicePolicyManagerTestCases --test 'com.android.cts.devicepolicy.MixedProfileOwnerTest#testAlwaysOnVpnUnsupportedPackageReplaced'
Test: cts-tradefed run cts --module CtsDevicePolicyManagerTestCases --test 'com.android.cts.devicepolicy.MixedManagedProfileOwnerTest#testAlwaysOnVpnUnsupportedPackage'
Test: cts-tradefed run cts --module CtsDevicePolicyManagerTestCases --test 'com.android.cts.devicepolicy.MixedManagedProfileOwnerTest#testAlwaysOnVpnUnsupportedPackageReplaced'

Change-Id: I477897a29175e3994d4ecf8ec546e26043c90f13