Change GC method in LogTest

A recent change in the System.gc() method made the GC test in LogTest
fail because Sessions were not being cleaned up correctly. The internal
Runtime.getRuntime().gc() method is now used to ensure the trigger of GC
for the purposes of the test.

Bug: 27230915
Change-Id: I2ff23c82343ebd65412c8a7e97a9866d2ee67a82