Generate app snippet and ask user confirmation for install

Once pre installation checks are complete, compute the install confirmation details viz install message, app icon, app label, update status and ask the user confirmation to proceed with the install. Register a listener to get user response from fragments to the controlling activity.

Bug: 182205982
Bug: 310284825
Test: builds successfully
Test: No CTS Tests. Flag to use new app is turned off by default

Change-Id: I18de3b996a71c9902bf7b09aecda5d267524a7d3