ANDROID: dm: verity: add minimum prefetch size

- For device like eMMC, it gives better performance to read more hash
  blocks at a time.
- For android, set it to default 128.
  For other devices, set it to 1 which is the same as now.
- saved boot-up time by 300ms in tested device

bug: 32246564

Change-Id: Ibc0401a0cddba64b862a80445844b4e595213621
Cc: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Keun-young Park <keunyoung@google.com>