Remove UTC offset from database. Fix clear history.

Bug: 32947084

Also add (currently unused) timestamp index.

The higher level "clear history" code needs work. This seems to work,
but hasn't been thought through very well. It was added mostly so
that I could actually test the ExpressionDB code.

This may require users to clear app data, but I seemed to get away
without it. You may just end up with an extra column if you don't.

Change-Id: Ibd2401a19d73c4824f549befd65a5c63633a894b