GestureConverter: initialize mThreeFingerTapShortcutEnabled

While it looks like this was being consistently initialized by
TouchpadInputMapper::reconfigure during real-world usage, in tests it
wasn't always being initialized, leading to a ubsan error about it
having an invalid value when I was working on adding some new tests:

    .../GestureConverter.cpp:276:9: runtime error: load of value 190,
    which is not a valid value for type 'bool'

Test: m
Test: run tests defined in subsequent CLs
Bug: 372571823
Flag: EXEMPT bug fix
Change-Id: I44798c35c38c2cb912a995002c6ee325d7bda167