Fix Preferences regression.

This change fixes reading filesystem-saved
java.util.prefs.Preferences.

Due to a bug in XmlSupport, when reading the
filesystem-saved preferences, the reader was
throwing an exception. The Android
implementation of xml parser adds additional
nodes that were not Elements, this was
causing a cast exception (the old code was expecting
only Elements).

FileSystemPreferences class was swallowing exceptions if
they happen during preferences content reading.
This changes adds additional logging to prevent this.

Bug: 27645233
Change-Id: Ieebd8a6a1002111654005bfcc42009b01c89ea70