Fix character count bug and Javadoc typos in SMS (with test cases)

Fix a character count bug I discovered while working on related SMS
bugs. Includes a new set of test cases to verify the fix for the
buggy calculateLength() methods ("runtest frameworks-telephony").

You can also verify that the counter works properly in the Mms app
by typing characters until the boundary is crossed where an
additional message part is required. The counter should count down
to 0 characters remaining before increasing the message count.

Change-Id: I4de68b82dfc53dcae094865798f2c0235a355d43