Add network agent to enhance NetworkProvider test

Add network agent to test more situation that could get the
onNetworkRequested callback.

Bug: 153614605
Bug: 153613690
Bug: 153612373
Test: atest CtsNetTestCasesLatestSdk:android.net.NetworkProviderTest
Change-Id: I7f827710b47546bd4419cc1ff06f03ec4635583d
Merged-In: Id494a1697cc1b73e8e56ae585a69faec31c59f52
(cherry picked from commit 9e92e57f)