Prevent bearing snap ease interfering with new ease

[andycalder]

This is a fix for #9793

In short, easeTo or flyTo may interrupt handler driven motion when the map's bearing is within the bearingSnap range. This triggers a bearing snap easeTo at the same time as the desired easeTo/flyTo. The camera can't handle both of these and it freezes and errors.

Further explanation of the chain of events that cause this bug is here #9793 (comment)

This PR:

• Adds a new argument allowEndAnimation to HandlerManager#stop to prevent a bearing snap interfering with a new easeTo or flyTo.
• Removes unused property _bearingChanged of HandlerManager.

Launch Checklist

• briefly describe the changes in this PR
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Fixed a bug where bearing snap animations interfere with new easeTo and flyTo animations and freeze the map</changelog>

[ryanhamley]

This seems ok to me but the gesture handling code isn't my area of expertise. @ansis can you give this a look to make sure there won't be any side effects to this change?

[ansis]

This looks good to me! thanks for your contribution