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>

src/com/android/settings/tts/TtsEnginePreference.java

@@ -30,6 +30,7 @@ import android.widget.RadioButton;
 
 
 import com.android.settings.R;
+import com.android.settings.Utils;
 
 
 public class TtsEnginePreference extends Preference {
@@ -136,6 +137,9 @@ public class TtsEnginePreference extends Preference {
         // Will be enabled only the engine has passed the voice check, and
         // is currently enabled.
         mSettingsIcon.setEnabled(isChecked && mVoiceCheckData != null);
+        if (!isChecked) {
+            mSettingsIcon.setAlpha(Utils.DISABLED_ALPHA);
+        }
         mSettingsIcon.setOnClickListener(new View.OnClickListener() {
             @Override
             public void onClick(View v) {
@@ -169,7 +173,12 @@ public class TtsEnginePreference extends Preference {
         // case mSettingsIcon && mRadioButton will be null. In this case
         // getView will set the right values.
         if (mSettingsIcon != null && mRadioButton != null) {
-            mSettingsIcon.setEnabled(mRadioButton.isChecked());
+            if (mRadioButton.isChecked()) {
+                mSettingsIcon.setEnabled(true);
+            } else {
+                mSettingsIcon.setEnabled(false);
+                mSettingsIcon.setAlpha(Utils.DISABLED_ALPHA);
+            }
         }
     }