Allow getting/setting app exclusion list for specific vpn profile

Provide a way for the system Setting app or other similar system
apps to set the app exclusion list for the specific VPN from
VpnManager. The list affects only VPN provisioned from
VpnManager to keep the design consistence for app using
VpnService.

The list is stored in the keystore for its persistence.

Bug: 192078259
Test: atest FrameworksNetTests CtsNetTestCases
Change-Id: I157823866e1899b40aa36ea2c0bc4a80370108be