Fix incorrect acceptableInput value on construction.

The value for acceptableInput was being calculated every time the
property value was read. This can lead to situations where the value
returned from successive property reads is different even though no
acceptableInputChanged() signal is emitted between the two calls.

This can be seen during QML component construction where emission of
the changed signal is suppressed until the component completes and the
value of acceptableInput changes as the other properties are set. If
the property is read during binding evaluation an intermediate value
can be seen and the QML engine will not re-read the property until
after the changed signal is emitted. This doesn't happen until the true
value of the acceptableInput property is toggled.

Fixed by changing the property getter to returned the precalculated
value of acceptableInput and ensuring that this value is set when
correctly.

Change-Id: Id3ba3a34988ff50f590e4f8330b873f390eaa025
Reviewed-by: Martin Jones <martin.jones@jollamobile.com>