Make only the "Add Network" WifiDialog fullscreen.

Fixes an issue where other instances of WifiDialog were being launched
fullscreen. Creates static methods for creating fullscreen and modal
WifiDialogs to make the style more explicit.

Bug: 63889135
Test: make -j40 RunSettingsRoboTests
Change-Id: I2200b5d7f817b9f69a6abb73bf2c04ea24556d19