Fix for affordances to add a mobile subscription

This changes the behavior for when we show an affordance to add a mobile
subscription via the eSIM manager (if eSIM is supported).  On the
Network & internet page, the behavior we now want for the "Mobile
network" pref has several possibilities as detailed below.

No existing subscriptions:
- Summary is "Add a network"
- Tapping on pref leads to the add subscription flow

One existing subscription:
- Summary is the name of the subscription (usually the carrier name)
- Tapping left-hand side of the pref leads to mobile network details
- A "+" button is shown on right of pref, and tapping "+" leads to the
  add subscription flow

2 or more existing subscriptions:
- Summary is "<count> SIMs"
- Tapping left-hand side of the pref leads to a new page with the list
  of subscriptions
- A "+" button is shown on right of pref, and tapping "+" leads to the
  add subscription flow

The existing code controlling the "Mobile network" pref (in
MobileNetworkSummaryController.java) already had behavior similar to
this, but needed to be updated to no longer rely on whether we're in
DSDS mode.

Also, the page with the list of subscriptions that you can go to in the
"2 or more existing subscriptions" case mentioned above has an "Add
more" link at the bottom which leads to the same eSIM manager flow, but
it was always showing this link even if the device doesn't support
eSIM. This CL also fixes that problem.

Bug: 127870567
Test: make RunSettingsRoboTests
Change-Id: Ie8cca47ac81f8992fa6ecf1940bdfed411fd333b