Improvements to ListFragment.

Now deals correctly with a content view containing just a list,
and adds a lot more built-in functionality: ability to show
custom text for an empty list, and indeterminant progress while
populating the list.

In addition, reworks transaction committing to be more aggressive
about committing the transactions as the containing activity
moves between its states (rather than waiting for the activity's
handler to process the transaction message whenever that may
finally happen).  And fixed a bug with saving/restoring state of
transaction replace operations.

Change-Id: I9617a0c4f248b50a61b319910323639b6de24f73