App bubble bar bubbles crash fix

* Added @Nullable annotation to `BubbleInfo.shortcutId` property and getter
* Changed `BubbleInfo` initialisation in `Bubble` class to account for nullable shortcut info

Test: atest BubbleTest
Flag: not needed
Bug: 279029374
Change-Id: Ic16cfb249c977e47e570d5e5bd000d5ffeb6ecdd