Enable AT-SPI by default for suppressed screen-reader

Enable AT-SPI by default for suppressed screen-reader

The ELM_SUPPRESSED_SCREEN_READER is enabled if the application wants to
speak directly without going through the screen-reader.

In an environment where the ELM_SUPPRESSED_SCREEN_READER is enabled,
the screen-reader does not use "IsEnabled" property but only
"ScreenReaderEnabled" property.

The "IsEnabled" is enabled by another AT-client such as the Aurum.

So this patch makes the AT-SPI bridge be enabled only if the "IsEnabled"
property is enabled, and ignore the changes of "ScreenReaderEnabled".

Change-Id: I9a006b559083ed19eb6985d7faa75ffac6edc3e5