phonewm: refactor Talkback shortcut logic into a controller

Wrap talkback shortcut related logic into one
`TalkbackShortcutController` class. This makes it easier to test
triple press logic since its using talkback shortcut.

Note: this is pure refactor. No behavioral change.

Test: atest WmTests:StemKeyGestureTests
BUG: 311006348
Change-Id: Icbd83958406e30a7bf85e5352bcbdfdcb68d8eaa