Replace PendingIntent @hide APIs

Connectivity service module is using some PendingIntent @hide
APIs but they are not able to call after CS becomes a mainline
module. Thus, replace them with similar System APIs.

Bug: 170593746
Test: atest FrameworksNetTests
      Manually test notifications can show normally.
Change-Id: I92ab7216f5fd46d449b6d8f80d3441f8966ac890