fix(ngOptions): do not unset the `selected` property unless necessary

Previously, when updating the value of a `select[multiple]` element, all options
were first set to `selected = false` and then the selected ones were set to
`true`. By setting an already selected option to `selected = false` and then
`true` again - essentially unselecting and reselecting it - caused some browsers
(including Firefox, IE and under some circumstances Chrome) to unexpectedly
scroll to the last selected option.

This commit fixes it by ensuring that the `selected` property is only set if its
current value is different than the new one and even then it is set to its final
value at once (i.e. without first setting it to `false`), thus avoiding the
undesirable behavior.

Fixes #15477

Closes #15478