Add tests for NATT keepalives stopped internally in KeepaliveTracker.

Add tests where NATT keepalives are stopped and cleaned up within
KeepaliveTracker in these scenarios:
1. startNattKeepalive is called with an invalid interval.
2. The state is STARTING and the network agent returns an error event.
3. The keepalive started successfully but becomes invalid and
   handleCheckKeepalivesStillValid is called on the network.

Bug: 281646074
Test: atest FrameworksNetTests
Change-Id: Iadf9f06030cae20f36d950f520c8dcc4181924ad