Add createVirtualKeyboard API

Adds a new system API guarded to allow creating a virtual keyboard.
Access to API is guarded by the privileged permission:
INJECT_KEY_EVENTS.

Design Doc: go/a11y-al-pk-dd

Bug: 397497164
Bug: 402581709

Test: Local tests with separate IME
Flag: com.android.hardware.input.create_virtual_keyboard_api

Change-Id: I9e6de531665967b3e5c34c4fe5d8e5ad2df81bd8