Fix talkback issue of Caption preference

Talkback speaks "double-tap to activate" on the LayoutPreference in the
Caption preference page, but it doesn't have any action after double
tap. So it should be set to unclickable. Set android:selectable to
false to fix it.

Fixes: 141900530
Fixes: 141901515
Test: launch TB and test it by manually
Change-Id: I82720dc46085b84d188730545061269d110f9a7c