Fix some problems with the "Mobile network" preference

This fixes a few issues with the "Mobile network" preference that shows
on the Network & internet page:

-The pref should not be available at all if the device is wifi-only.

-The pref should be disabled in airplane mode.

-If eSIM support is disabled, we need to disable entry points for adding
 a new eSIM subscription (the summary and click handler in single-SIM
 mode with no physical SIM, and the 'add' button in dual-SIM mode).

Bug: 116349402
Test: make RunSettingsRoboTests
Change-Id: Ic6fe426e9f890d80b399d48a2c9984d4d8b02a10