Move letterbox task positioning to LetterboxTaskListener.

Changes:
- Introduce LetterboxTaskListerner
- Pass activity bounds and task position from WM Core to WM Shell
- Position a task from LetterboxTaskListener.

Fix: 170210431, 170213038, 168706428, 171549361
Test: go/wm-smoke
Test: atest ShellTaskOrganizerTests, LetterboxTaskListenerTest
Change-Id: I9c25b7fe1053900fe3b6eea8e71db302a0c14a5a