Add signal strength icons to multi-SIM header

When a device is in DSDS mode with multiple SIMs, we show a header on
the Network & internet page with a "connectivity overview", with an
entry for Wi-Fi (if connected) and each active SIM. The icon for Wi-Fi
shows signal strength, but the icons for the active SIMs were just
static; this CL fixes that. It introduces a class for listening to
signal strength changes on a set of subscriptions, and moves some
existing code we had for displaying signal strength (when manually
picking networks for GSM) in NetworkOperatorPreference into a utility
method that can be shared.

Bug: 128855095
Test: make RunSettingsRoboTests
Change-Id: I668cafe1e9f9c3651b1f33783c8538ad9c2732b5