ContextHubTestModeManager: Reduce event probabilities

This CL reduces the probability that a duplication
or drop event occurs during the reliable message
test. This CL should fix failures where the events
were occurring too frequently, so even though the
code was working correctly and retrying messages,
all messages were being dropped leading to test
failure.

Bug: 333567700
Change-Id: I1fe7ed7bb0fbdfb755d59895c514c59111af1c42
Flag: TEST_ONLY
Test: Presubmits