Fix home/back buttons flow in PreferenceActivity.

Bug: b/35977048
Test: Done manually. Automated test will be added later.

After recent changes in PreferenceActivity finishing the activity on
pressing back is not correct anymore. Instead onBackButton method should
be called to deal with it (we have fragments transactions for instance).

There can be multiple buttons:
1. The native default one - calls onBackPressed by default so no extra work
needed.
2. PreferenceActivity has buttons that can de displayed when settting
EXTRA_PREFS_SHOW_BUTTON_BAR to true. But it is a private API and I
didn't find any usage of it, so there is no need to change that. Also
it looks like something very specific used in WiFi wizards.
3. Apps often use a toolbar that has a home button which essentialy
emulates back button. The issue is that this Activity never had support
for this. So apps had to override onOptionsItemSelected and some of
those are unfortunately calling finish() which worked previously when
PreferenceActivity wasn't using fragments in single pane. This CL adds
support so app will no longer need to override that method and some
apps need to be instructed to use that override in O.

Change-Id: I94f557edb2e03f89fbf02e800beb390b2ad0544f