Fix detection of CDMA CMAS cell broadcasts.

CMAS cell broadcasts do not necessarily set the message
priority indicator subparameter. Interpret the user data as a
CMAS message if the service category is in the CMAS range.

Includes a new test case in CdmaSmsCbTest to verify correct
decoding of an actual CMAS monthly test message.

Bug: 6506785
Change-Id: I2a26a5194a1a02e34c1ef862ff348e1571df51c4