Fix accesses to MmsSms.CONTENT_CONVERSATIONS_URI.

Because one of the columns was duplicated between MMS_ONLY_COLUMNS and
SMS_ONLY_COLUMNS, the last entry of UNION_COLUMNS was null, leading to
a NPE:

E/DatabaseUtils( 2728): java.lang.NullPointerException
E/DatabaseUtils( 2728):         at
android.database.sqlite.SQLiteQueryBuilder.buildUnionSubQuery(SQLiteQueryBuilder.java:540)
E/DatabaseUtils( 2728):         at
com.android.providers.telephony.MmsSmsProvider.getConversations(MmsSmsProvider.java:815)
E/DatabaseUtils( 2728):         at
com.android.providers.telephony.MmsSmsProvider.query(MmsSmsProvider.java:315)
E/DatabaseUtils( 2728):         at
android.content.ContentProvider.query(ContentProvider.java:857)
[...]

Change-Id: I9b08f37c40b0d4196900cdb169b0ec2570c875f1