Correct attachment type for Mms only has subject

Mark the attachment type of Mms only has subject as TEXT
avoid to show Mms View as untimely

Change-Id: I346ae4af4983e9e8dbe9533eeddef445affc339a
Signed-off-by: Roger Chen <cxr514033970@gmail.com>
Mms: Add null check for mmp

CM devices that set config_mms_content_disposition_support to false create an instance where pdu
headers are ignored.

See:
https://github.com/android/platform_frameworks_base/blob/master/core/res/res/values/config.xml#L804

Change-Id: I33b7c6878468208867a9e96133655cb7109f6415