Add getLegacyVpnInfoPrivileged method

Follows the Google's permission check policy.
Add a getLegacyVpnInfoPrivileged method which skips the permission check
and change getLegacyVpnInfo to check the permission and then call getLegacyVpnInfoPrivileged.
It is already checked in this commit : https://android-review.googlesource.com/#/c/141771/



Signed-off-by: SangJin Cha <sj.cha@lge.com>
Change-Id: I83cfaedbd85745574f3ddf118b11b6e0415483c6