Refactored NetworkPolicyManagerService mUidRules.

NetworkPolicyManagerService (NMPS) keeps an internal list of uid
rules (mUidRules) for network restrictions, and when these rules
changes it needs to notify external listeners (such as
ConnectivityService / CS).

Prior to Android N, both Data Saver mode (the feature previously known
as "Restrict Baground Data") and Battery Save mode used the same set of
firewall rules to implement their restrictions: when Battery Saver mode
NPMS would mark all networks as metered and set the proper firewall
rules externally.

Recently, these 2 modes were split in 2 distinct firewall rules and
NMPS.updateRuleForRestrictBackgroundLocked() was changed to update
the mUidRules logic based on the Data Saver firewall (since the Battery
Saver firewall changes are handled externally, on
updateRuleForRestrictPowerLocked()). As such, CS was not notified when
the power-related changes were made, which would cause apps to get a
state of CONNECTED / CONNECTED when querying its active connection.

This change refactores the mUidRules to use bitmasks, in preparation for
another change that will fix the issue.

It also fixes a minor bug that was preventing removed packages to be
removed from the whitelist.

BUG: 28521946
Change-Id: I9f0e1509a6192cad403f740c1cd76a6b7dab7d26