defaults: pulse: Use 2 fragments of 16 ms to reduce latency

The default of 4 fragments of 25 ms results in 100 ms latency, which is noticeably high, especially when moving audio between two cards (SoC and modem) as the latencies add up. Librem 5 appears to be perfectly capable of dealing with lower latencies, so let's reduce it to 32 ms.

This is a somewhat conservative choice, as there are lower values that still appear to work well, but going lower increases the chances of underruns happening under load, so we may want to wait until we scale DRAM frequency up dynamically to not regress on playing audio with screen locked use-case.