Add pre-defined apps to the restrict background whitelist.

System config can defined apps to be automatically whitelisted for
restricted background data, but the user can remove the whitelist.

Implementation-wise, NPMS now keeps a list of
<revoked-restrict-background> UIDs in the netpolicy.xml file, and when
it starts it compares the UIDs returned by SystemConfig against this
list, and only whitelist them if they are not revoked. The
revoked-restrict-background is then updated as users change the
whitelist status of UIDs.

BUG: 27366993

Change-Id: I427024fd058924fc9831e409da6636e1bf8e4219