Remove assertEventuallyTrue as it's unnecessary.

The calls in testAvoidBadWifi are no longer necessary now that
waitForIdle is reliable, and the calls in testPacketKeepalives
are replaced with a wait for the NetworkAgent to disconnect.

Test: ConnectivityServiceTest passes 100 times in a row.
Bug: 32561414
Change-Id: Icbb161ca6e343bd14764a1c9ccfdd14b6cd6803f