Bind to the recommendation service specified in the Setting.

High level changes to NetworkScorerAppManager:
  * Implemented getAllValidScorers() and removed the old
    config-based discovery code.
  * Implemented setActiveScorer() to persist its given package
    name to Settings if it represents a valid network
    recommendation app.
  * Added a new method that reverts the setting back to the
    configured default if the current setting represents an
    invalid app.

High level changes to NetworkScoreService:
  * Updated the PackageMonitor to only watch a single package.
  * Moved most of the startup logic to onUserUnlocked() so we
    don't have to worry about whether or not the device is encrypted
    when querying the PackageManager.
  * The PackageMonitor is only registered/unregistered when the
    package it's watching changes.

Test: runtest frameworks-services -c com.android.server.NetworkScorerAppManagerTest
Test: runtest frameworks-services -c com.android.server.NetworkScoreServiceTest
Bug: 35095406
Change-Id: Ib32aca72dac4b831a64ceb3cd5c31e8fa2f61396