Add multipath preference, background status API

Add APIs for getMultipathPreference and getRestrictBackgroundStatus.
Both are used by Connectivity to back the external
ConnectivityManager.getRestrictBackgroundStatus, and
ConnectivityManager.getMultipathPreference APIs.

Test: atest CtsNetTestCases
      atest ConnectivityServiceTests
      atest NetworkPolicyManagerServiceTest
Bug: 176289731
Change-Id: I8a03162b2f6691086bb64e75ffd354cdfca7f86a
Merged-In: I8a03162b2f6691086bb64e75ffd354cdfca7f86a