Add "smallestScreenSize" into homepage activities

The Settings home activity was initially launched in full-screen size,
and then resized in order to be displayed with another placeholder
activity size-by-side.

After ag/18668447, the smallest screen width dp configuration of the
activity was changed to half of the screen size (vs. the fullscreen
size) while embedding.

Therefore, the system updates the configuration changes using
CONFIG_SMALLEST_SCREEN_SIZE, which the Settings home activities didn't
declared. This makes Settings show the full-screen home layout in
dual-pane mode.

Fix: 235449822
Test: visual, robotest
Change-Id: I801ff5fb20b2a42c94223a6edd4af08b67e7f4e9