Only show 2-pane deep link when homepage is available

DeepLinkHomepageActivity is disabled by default and may be
enabled after receiving the PRE_BOOT_COMPLETED broadcast.

On some virtual devices, DeepLinkHomepageActivity may not
be enabled for test cases. This is a fallback option to
show a full screen Activity to pass tests.

Bug: 221149428
Test: make RunSettingsRoboTests ROBOTEST_FILTER=SettingsActivityTest
Change-Id: I4bbe785176fe3fe8831484141aff7367e4ca25ce