init: chroot from recovery to /first_stage_ramdisk

When using the recovery image as a trampoline to boot the system,
first chroot from the recovery image to /first_stage_ramdisk, to
minimize differences between these two boot paths.

Primary motivation is due to the fact that the basename of each mount
point is used by device-manager to name its nodes, and the previous
code that created used /system_recovery_mount as the mount point for
system.img broke AVB.  Instead of hacking around that issue, this
change unified mounting for the recovery trampoline and true first
stage ramdisk paths.

Bug: 114062208
Test: AVB works with blueline_mainline
Change-Id: Iffb154962b6e160150917e068f1e7d0bf7cb84e7