Add getSummaryMethod to AbstractPreferenceController

We want a dedicated getSummay method, as summary text
is a fairly important part of the preference framework.

Specifically for preferences with dynamic summaries, having a
method to provide summaries for Slices, and in the future, search
is necessary for correctness.

Change-Id: I435ccab7758d90515583fd8ca10a9b1ef0c858b9
Bug: 71514936
Test: make -j40 RunSettingsRoboTests