Desktop screenshots: Open UI for partial screenshot keyboard shortcut

When the keyboard shortcut for partial screenshots is entered, the
pre-capture UI will open. Note: there is still follow-up work needed to
default select the partial screenshot option in the resulting UI
toolbar.

Bug: 420714826
Test: atest ScreenCaptureKeyboardShortcutInteractorTest
Test: atest SysUIKeyGestureEventInitializerTest
Flag: com.android.systemui.large_screen_screencapture
Change-Id: Ia258b7f0146da58fbb8e23567389d0d573661aa0