Adds a section for unaccounted data.

Often sub-directories of /data are populated with data framework has no
control of (for example, by the device's specific dumpstate_board()
function), and that data is now available through
MeasurementDetails.getUnaccountedSize().

BUG: 30952263

Change-Id: I8d3fda6668f2a57ee025fb135b33a244167d1e4b