Make positioning in letterbox mode configurable.

Changes:
- Allow to specify in config_letterboxPortraitGravity and config_letterboxLandscapeGravity desired alignment for letterboxed apps.
- Add ADB commands to allow overriding these config values for testing
- Move positioning logic for letterbox mode from WM Core to WM Shell.

Test: atest LetterboxTaskListenerTest LetterboxConfigControllerTest
Test: go/wm-smoke
Test: manual with adb shell dumpsys activity service SystemUIService WMShell (get)set-letterbox-portrait-gravity / (get)set-letterbox-landscape-gravity
Fix: 170216257, 170215849
Change-Id: I8e6392faab05bec1bf61b9691f3bd9b8b09b9621