Reduce memory leak in NetworkMonitorTest

This change includes:
 1. Fix thread leak of probing threads. This won't impact
    production code since probing threads would be naturally
    terminated after probe timeouts.
    But this will impact tests since the threads are not
    terminated when leaving the tests, causes NPE when
    accessing resources which are no longer exist.
 2. Clear inline mocks which hold the references of test
    instances and caused 32MB of memory leak.
 3. Add @MonitorThreadLeak annotation in the test to ensure
    no thread leak problem in the future.

Test: atest ConnectivityCoverageTests:com.android.networkstack.com.android.server.connectivity.NetworkMonitorTest
      (With memory leak detect patches applied)
Fix: 321606002
Change-Id: I7b88b9da7c8fe93ef17d116ae83c29872f8094eb
2 files changed