Add VPN app into power saver whitelist for a while to start its service

If the VPN app is not launched, then its background service
cannot receive the VpnManager events.
Add VPN app into power saver whitelist for a while can let its
background service to receive VpnManager events.

Bug: 234104087
Test: Confirm that the VPN app has received VpnManager events.
Change-Id: I9819a78ec0cbbd6879baae645ef42c22756b09d4
(cherry picked from commit 62991fc4)
Merged-In: I9819a78ec0cbbd6879baae645ef42c22756b09d4