Integrate band selection into the files app.

This consists of:
1) Moving both BandSelect* classes into MultiSelectManager as nested
classes. This removes extra layers of indirection and makes code easier
to comprehend.
2) Fixing an issue where band selection could start within an item
instead of only within margins between items.
3) Adding "provisional selection" support to the Selection class. This
gives band select the ability to select some items "temporarily" such
that when the band select no longer covers those items, they are
deselected.
4) Adding glue code between the classes.

Bug: 23081429, 23083146, 20669231

Change-Id: I0dc57e2c0d2ccedb3e1218f0e496de637be227a2