dynsystem: Enlarge default shared memory size and allow size override

This modest adjustment 8KiB -> 64KiB significantly boosts the DSU
installation time:

* physical device: 2m34s -> 45s
* virtual device: 46s -> 30s

Also make the shared memory size customizable for fine-tuning.

Bug: 225310919
Test: start a Dynamic System installation and check its execution time.
Change-Id: I0418ba19c3824d31feafce54a71c054061d010e8