Always reserve space for preference icons.

This aligns text on preferences on the same screen when some of them
don't have icons.

- Use a custom layout for
  preference
  EditPreference
  switch preference
  VolumeSeekPreference
  DialogPreferencei
  Restricted*Preference
  DropdownPreference

- Moved related style defs into a new file styles_preference.xml,
  because current styles file is too large to read.
- Switch <PreferenceScreen ...> to <Preference ...> for a bunch of
  regular preferences.
- Also removed some dead style.

Bug: 33430928
Test: runtest --path packages/apps/Settings/tests/app/src/com/android/settings/dashboard/PreferenceThemeTest.java

Change-Id: Ic9b75d3f133f2a83cd999297b8596253d4b10161

res/xml/date_time_prefs.xml

@@ -43,7 +43,7 @@
         android:summary="12:00am"
         />
 
-    <PreferenceScreen
+    <Preference
         android:fragment="com.android.settings.ZonePicker"
         android:key="timezone"
         android:title="@string/date_time_set_timezone"