Add logic for building content description for pressed keys

added logic to build custom content description when user is customizing
shortcut. This allows us to pass content description to our custom
text/icon box composable. Also this re-uses logic for building content
description in shortcut helper ensuring consistency across shortcut
helper.

Fix: 394480347
Flag: com.android.systemui.keyboard_shortcut_helper_shortcut_customizer
Test: ShortcutCustomizationViewModelTest
Change-Id: I409923ed219b878e62c5ccf94771ed379e89e946