Add logging for Dream swipe up to show Bouncer.

Tested the behavior both in existing tests and in the new
testSwipeDownPositionAboveThreshold_expandsBouncer_doesNotLog().

Also removed the existing
testExpansionAmount_whenBouncerShown_setsCorrectValue(), because it
duplicates testCollapseAmount(). And updated some of the existing tests
to have more explicit names and comments.

Bug: 213906448
Test: Manually on device via `statsd_testdrive 90`
Test: atest BouncerSwipeTouchHandlerTest

Change-Id: I217025134d7574c3ce1d123688ae0db4ceef848c