As far as I know, the word askari standing by itself does not necessarily mean "police officer", though it can. According to Kamusi ya Kiswahili Sanifu: askari  mtu anayelinda na kuihami nchi; mwanajeshi;  mlinzi: askari polisi, askari jela, askari magereza, askari wa mgambo; askari wa kukodiwa: watu wanaokodishwa kupigana kwa ajili ya faida za kibinafsi tu. My personal choice would be to say askari polisi or afisa/ofisa wa polisi to mean "police officer." Also, the word polisi by itself can mean policeman.
We were told earlier that Askari means soldier. Is a soldier and a police officer the same person in Tanzania? I know this is the case in countries under military rule and is called martial law. But, Tanzania is a democracy. I guess they can set it up how they want to.