Adding the ability to customize the subtitle on an unavailable slice

When a slice is depending on some setting and the setting is off,
it shows "depends on another setting".
Add an new attribute for Preference to customize the subtitle when
a slice is unavailable.

Bug: 118399193
Test: Robo test on com.android.settings.slices, com.android.settings.core
Change-Id: I84a8400295b36abb357e5baf98e9be3a8d6ea897