Add APIs for adding custom input gestures(keyboard shortcuts)

DD: go/customizable_shortcuts
PRD: go/custom-kb-shortcuts

Part 1 of chain of CLs for customizable shortcuts:
- Adds APIs
- In memory storage for custom input gestures

Bug: 365064144
Test: atest InputTests:CustomInputGestureManagerTests
Flag: com.android.hardware.input.enable_customizable_input_gestures
Change-Id: I3578226db9a740db5f44457432e88aef94f12d4e