Introduce config_disable_uninstall_update flag.

This adds the following flag:
config_disable_uninstall_update

Which by default is false. When set to true, it will hide the "Uninstall
updates" menu item for all cases.

This is useful for cases where the device OEMs want to allow users to
install apps, but do not want the ability to roll back updates on system
applications.

Bug: 62379281
Test: make RunSettingsRoboTests ROBOTEST_FILTER=AppInfoDashboardFragmentTest
Change-Id: I55fe92467ca95c05f6682174c117031d8295790e