Moved core startable logic out of repository

As part of go/optimizing-shortcut-helper-component-instances
refactoring ShortcutHelperStateRepository to do only one thing- handle
shortcut helper state, while ShortcutHelperCoreStartable, handles any
work related to starting up ShortcutHelperComponents, such as
registering broadcast listeners.

Test: this is only a refactor, existing tests cover this.
Flag: com.android.systemui.keyboard_shortcut_helper_rewrite
Bug: 385325834
Change-Id: Iedb1315aaf503e0e2102a6ac2cfcd90b7cc24fda