Add HistoryTracker to FalsingManager

This institutes a HistoryTracker mechanism that will allow the
FalsingManager to take recent activity into account when classifying
gestures. The HistoryTracker returns a penalty that is pased to each
classifier for them to use when making decisions.

As of right now, no classifier actually uses the history penalty when
making a classification decision.

The HistoryTracker keeps records of the last 3 seconds of falsing
results, applying a decaying penalty to the results over time. It also
assigns a confidence to its results, based on how consistent they are.

Each individual classifier is also able to return a confidence score
now, such that, if they are not confident in their decision, they
don't throw off the score that gets stored in the HistoryTracker.
Confidences currently returned by the classifiers are somewhat
arbitrary and should be adjusted in future CLs.

Everything is currently open for adjustment:
- The length of history
- The decay function applied to history
- The method by which our confidence in the history is calculated
- The confidence returned by each individual classfier
- How the history penalty is taken into account by each classifier
- ... and probably more.

Bug: 172655679
Test: atest SystemUITests
Change-Id: Ief2a5144a0cda659c7d78ed6e489231d481b6872