ConnectivityServiceTest: fix flakyness

This patch attempts to fix the remaining spurious failures in
ConnectivityServiceTest, which have two causes:
 - waitForIdle() does not take into account the NetworkAgents handlers.
 - the deadlines in testRequestBenchmark are sometimes exceeded.

To fix the first issue, waitForIdle() is moved to a test level instance
method and also calls waitForIdleHandler on any non null
MockNetworkAgent. This is expected to fix spurious errors for the
following tests:
  - testMobildeDataAlwaysOn
  - testLingering
  - testPacketKeepAlive
  - testMMSonWiFi

To fix the second issue, the deadlines for testRequestBenchmark are
extended by 10ms. Also, the failure message is made more actionable by
providing the total time it took for the operation, instead of printing
the number of dispatches that were achieved before the deadline.

Bug: 32561414
Test: tests pass many times in a row (~500).
Change-Id: Id33c6ac1edfb0b89634fa7789dccb2da237e2709