[AT-SPI] Use Accessible::IsHighlighted()

[AT-SPI] Use Accessible::IsHighlighted()

This helper allows to simplify multiple comparisons of Self() to
GetCurrentlyHighlightedActor() to simply checking IsHighlighted().

Some new usages of IsHighlighted() are also introduced before event
emission in order to reduce D-Bus traffic.

Change-Id: I5d8906db22c84764de3e8df631df4d4cf8062c2f