Added API to whitelist apps for background restriction.

The new APIs are:

    void addRestrictBackgroundWhitelistedUid(int uid);
    void removeRestrictBackgroundWhitelistedUid(int uid);
    int[] getRestrictBackgroundWhitelistedUids();

The whitelist is persisted in the same XML used for the other policies,
using the new 'whitelist and 'restrict-background' tags. Examples:

3 whitelisted apps:

<whitelist>
  <restrict-background uid="10001" />
  <restrict-background uid="10009" />
  <restrict-background uid="10086" />
</whitelist>

No whitelisted app:
<whitelist />

BUG: 26451391

Change-Id: I01b114c346fd8abc05433ad43d010379384f8c2b