GuidedStepFragment: made Picker accessibility friendly

Interpreting keys in Picker view does not work with accessibility services.
Talkback 4.4 expects the view to be focusable to drive the UI.  The change
makes Picker's child views focusable.  Also added new functions to
block focus out of item view when it's activated for editing.

Bug 26725708

Change-Id: I332fe577dffa4fe86a83dfe25e13cb90444d5da3
9 files changed