Allow clients to register for listening to changes in capabilities.

Clients can register for listening to changes in procstates and if
they are only interested in knowing if the procstate crosses a certain
threshold, they can specify a "cutpoint" and they will only get
a callback if the procstate crosses that "cutpoint" but clients cannot
use this if they are also interested in changes in capabilities. So,
add a way for clients to use the "cutpoint" facility and also listen
to changes in capabilities.

Bug: 177641226
Test: atest services/tests/servicestests/src/com/android/server/am/ActivityManagerServiceTest.java
Test: atest services/tests/servicestests/src/com/android/server/am/UidObserverControllerTest.java
Test: atest ./tests/cts/hostside/src/com/android/cts/net/HostsideRestrictBackgroundNetworkTests.java
Change-Id: I1198e602ca81382975500b10f930dacfd16919e3