fix(android): key tip sticky with multitap

Fixes #6489.

The issue arose in multitap scenarios, where the tip display state
became desynchronised with the actual display state as maintained in
the Android code.

The fix I applied was to reduce some of the state checks into the js
wrapper android-host.js. This is not ideal. The state machine for key
tip display is scattered across four functions in three files:

* visualKeyboard.ts: VisualKeyboard.highlightKey
* visualKeyboard.ts: VisualKeyboard.showKeyTip
* embedded/keytip.ts: KeyTip.show
* android-host.js: oskShowKeyPreview / oskClearKeyPreview

I have moved the actual re-creation check for the tip to the lowest
level functions -- those in android-host.js. But a more detailed look at
the state machine and a refactoring would certainly be worthwhile.