XML code completion improvements

XML code completion improvements

First, fix code completion for @android: resources such that if you
type @android: the various resource types (@android:drawable/,
@android:layout/, etc) are offered.

Second, fix completion for the @android: token itself such that if you
type "@a" then "@android:" is offered as a completion.

Finally, make resource name completion work even for attributes that
aren't marked in the metadata as allowing resource references.  This
will not be done for empty completion context, but if the user
-explicitly- types a "@" in the value field, then resource completion
will work. This is necessary for some attributes where our metadata is
wrong, such as android:minHeight, where code completion currently
refuses to complete a @dimen/ completion prefix.

Change-Id: I175c8f7230d56987b9a945a2b791a2eb3e018a7c