Touchpad: fix some NullAway warnings

Change I930e9fc85e09e03da3513e5f8443077fa8ea91b0 triggered some NullAway
warnings that are unrelated to the change, but some are trivial to fix,
for example by replacing member variables with local ones.

At the same time, add some `@NonNull` annotations to overriding methods
on Android Studio's recommendation.

Test: check touchpad gesture tutorial and pointer speed settings work
Test: $ atest SettingsRoboTests:com.android.settings.inputmethod
Bug: 245989146
Flag: EXEMPT refactor
Change-Id: I717eb2a2d68a46dce8ebaa95bef1183d09380f51