Register NPMS$UidObserver to listen for only necessary procstate changes.

Right now, NPMS listens for all the procstate changes. This will slows
down processing of procstate changes which actually matter and also
fills up the netpolicy history pretty quick.

Bug: 112485285
Test: cts-tradefed run singleCommand cts-dev -m CtsHostsideNetworkTests -t \
      com.android.cts.net.HostsideRestrictBackgroundNetworkTests
Test: atest services/tests/servicestests/src/com/android/server/NetworkPolicyManagerServiceTest.java

Change-Id: Iaaef8ea4ee16f5b69cafc1e7e5908c6ff6fe1ed3