Retains cached widget in launcher process

Currently cached widget are retained in LauncherWidgetHolder which is
released when Launcher activity is recreated. This CL moves the cached
widget into LauncherAppState to keep the cache alive.

Bug: 268189435
Test: steps below
1. Add multiple widgets (Calendar / Weather ... e.t.c) to Home Screen
2. Open Google Map, start navigation to any place
3. Google Map enters navigation mode and changes resolution
4. Swipe up to exit Google Map and go to Home Screen
5. Verify you don't see deferred widget host view.

Change-Id: I8b56167313780cd1be2a5da88517114acc6d44af