Fix infrequent comment in LayerHistoryTest

Fix some occurrences of outdated/incorrect comments saying layers turned
inactive, when actually they only turned infrequent. In the current
logic, layers can still be active when infrequent for other reasons.

Bug: none
Test: presubmit
Test: atest libsurfaceflinger_unittest
Change-Id: If65918a4732fda0d9c54412c7d7237f6e3812aa2