fix(select): make ngOptions support selectAs and trackBy together

This commit implements two functions, "isSelected()" and "getViewValue()"
to properly compute an option's selected state and the model controller's
viewValue respectively. These functions give proper precedence to "track by"
and "select as" parts of the ngOptions comprehension expression, which were
previously inconsistent and incompatible.

Fixes #6564
wip: simplify `select`