Save mInbox in OnePane

Fixes one cause of overlapping labels list.  Other issues remain:

1. mInbox can still be null for other reasons: launching from a widget
   or notification. In this case we need to instantiate a list context
   correctly. This is currently broken.

2. Fragment transactions can still cause overlapping views, since this
   issue was reported *before* we setRetainInstance to false.

b/7048724 Manage labels screen is overlapping on the conversation list view screen

Change-Id: Idfd97aac1cf24c8a072247a0fca8868c91f26bf9
1 file changed