Fix keystore demo layout for Nexus 7

The plaintext and ciphertext fields weren't showing up on Nexus 7
(possibly other devices too) because of some unresolvable dependencies
between UI elements.

Also stopped leaving these fields editable when no key was selected
and corresponding action buttons were switched off.

Change-Id: I32ed3e2f24c134504a02db6fdfb9905ec147fbdd

samples/ApiDemos/src/com/example/android/apis/security/KeyStoreUsage.java

@@ -129,6 +129,7 @@ public class KeyStoreUsage extends Activity {
         ListView lv = (ListView) findViewById(R.id.entries_list);
         mAdapter = new AliasAdapter(getApplicationContext());
         lv.setAdapter(mAdapter);
+        lv.setChoiceMode(ListView.CHOICE_MODE_SINGLE);
         lv.setOnItemClickListener(new OnItemClickListener() {
             @Override
             public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
@@ -250,6 +251,8 @@ public class KeyStoreUsage extends Activity {
      * enabled or disabled.
      */
     private void setKeyActionButtonsEnabled(boolean enabled) {
+        mPlainText.setEnabled(enabled);
+        mCipherText.setEnabled(enabled);
         mSignButton.setEnabled(enabled);
         mVerifyButton.setEnabled(enabled);
         mDeleteButton.setEnabled(enabled);