Renaming setting to navigationbar_key_order

This aligns the naming with an existing third party definition,
which will allow for effective migration between devices in the future.
As an additional measure, adds a pre-deprecated global definition for the setting.
It should be configured to automatically transfer any values to Secure.

Test: adb shell settings put secure navigationbar_key_order {0 or 1}
Test: run the command with the above values. The bar order should update.
Flag: android.view.accessibility.navbar_flip_order_option
Bug: 424829220
Change-Id: I79c2bc794a635ba69317d5d92c747001e720ed5f