Refactor ModifierShortcutManager.

Instead of arrays of intents to represent shortcuts, now use
data objects to make the code simpler and easier to follow.

Bug: 358603902
Flag: com.android.hardware.input.modifier_shortcut_manager_refactor
Test: atest ModifierShortcutManagerTests ModifierShortcutTests

Change-Id: Ibc368bc804d17ede6c83fbf77070e85776738e77