Replace Context @hide API in MultinetworkPolicyTracker

MultinetworkPolicyTracker is part of Connectivity mainline module
which cannot call @hide API Context#registerReceiverAsUser. Thus,
replace it to Context#registerReceiverForAllUsers.

Bug: 170634463
Test: atest FrameworksNetTests
Test: Manully check that receiving intent and unregistering
      receiver can work normally.
Change-Id: Ie22a3aa40bc9243082672a64dba7c7940f9ff21a