Screen capture: Setup keyboard shortcut for partial screenshots

Registers a keyboard shortcut (Meta + Ctrl + S) for taking a partial
screenshot. The resulting action for the keyboard shortcut will be
implemented in a separate CL.

Bug: 420714826
Test: atest KeyGestureControllerTests
Flag:com.android.hardware.input.enable_partial_screenshot_keyboard_shortcut

Change-Id: Ice5a2e8cd4e23869376e1165703f436f74d4f725