I didn’t test this code but i believe it should work.
int unroundedMinutes = isha_jamaat_cal.get(Calendar.MINUTE);
int mod = unroundedMinutes % 5;
isha_jamaat_cal.add(Calendar.MINUTE, mod < 3 ? -mod : (5-mod));
The basic idea is to check whether the current time is nearest to the next or the previous 5 minutes clock mark and adjusting the added value based on this.
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solved Rounding calendar time to nearest 5 mins [duplicate]