fs_mgr: Add support for at-boot verification

Running dm-verity on heavily accessed partitions leads to performance
slowdowns, especially on low-RAM and slow-CPU devices.

This patch introduces a flag to allow an entire verified partition to be
read once at boot, to check for corruptions. If the reads are
successful, we can mount the partition as raw & read-only, and if not,
we can revert to mounting it as a verity partition, just like before.

Usage of this flag will entail a slowdown of time-to-boot, but should
lead to improvements in runtime performance.

Bug: 32433608
Change-Id: I97717683a00ad6fa347e63b72b1a9bf1d2946315