Grey out settings icon when it is in disabled state

If engine is not active, its settings icon is disabled in
Settings > Language & input > Text-to-speech output screen.
Currently, settings icons for all TTS engines are shown at
the same opacity. This fix dims settings icons of not-selected
engines.
Make sure that more than one TTS engine are installed on the
target, for e.g., Google Text-to-speech Engine and Classic
Text To Speech Engine (SVOX Classic TTS).

Additionally, since setAlpha() is used in multiple places within
Settings package, moved DISABLED_ALPHA declaration to Utils.java
in order to have single point of reference.

Change-Id: Ifa7de79814a2f4a4aa021cd8621cbfab41655680
Signed-off-by: Shuhrat Dehkanov <uzbmaster@gmail.com>