It seems that an officer might develop RAS based on observation, which would provide for the stop. But to "prove" the act of texting the officer would have to search the device for "proof" of having been used to text in the time period when the officer made the observation providing him with RAS.
Since mobile devices can be used to download web pages, maps, music and other things not listed (they really should have defined the behaviors better) the claim could be made that one was not, for example, texting but selecting MP3 files, or content from web-streamed media, or looking up internal phone directory "contact" information - just to name a few possibilities that are not made illegal by the bill.
The officer would have to 1) sieze the device and 2) a warrant obtained to search the device for "proof" of having been used for texting at the time of the alleged offense. Otherwise, a defense is mounted by simply denying the alleged act and requiring the officer to submit proof beyond a reasonable doubt.
If the officer is close enough to see into a vehicle and personally observe the act of texting they are driving recklesly themselves. If they pull a driver over and upon approaching the vehicle observe the driver texting, no crime has been committed because the driver is parked.
Another "feel good" law that will not accomplish its stated goal of improving driving safety by eliminating one form of reckless driving except in such few cases as to be meaningless.
Further, if it is reckless for the proles, it surely is reckless for the Only Ones. (I've seen them at driver training.)