Fixed connectivity state in some restricted network scenarios.

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.

Another scenario that is not properly handled is when a UID whitelisted
for Data Saver is brought back to foreground: although the proper
firewall rules are set, CS is not notified, and the apps state would be
DISCONNECTED / BLOCKED.

This CL introduces many changes that fix this issue:

- Fixed updateRuleForRestrictBackgroundLocked() to invoke
  onUidRulesChanged() when the Battery Saver status changed.
- Fixed updateRuleForRestrictBackgroundLocked() to invoke
  onUidRulesChanged() when an app whitelisted for Data Saver is brought
  back to the foreground.
- Added a new API (onRestrictPowerChanged() and getRestrictPower())
  to notify external services about Battery Saver mode changes.
- Fixed CS logic to properly handle the Battery Saver changes.

Externally to this change, the CTS tests were also improved to verify
the apps get the proper connection state; they can be verified running:

cts-tradefed run commandAndExit cts -m CtsHostsideNetworkTests \
    -t com.android.cts.net.HostsideRestrictBackgroundNetworkTests

BUG: 28521946

Change-Id: I8eaccd39968eb4b8c6b34f462fbc541e5daf55f1