fs_mgr: avb should use built-in kernel command line parser

Switch from open-coded parsing of /proc/cmdline to built-in
fs_mgr_get_boot_config_from_kernel_cmdline function.

Test: compile and smoke boot test
Bug: 109821005
Change-Id: I935d2e76c76ad417cbcee398e2afbf9b061d1719
Merged-In: I935d2e76c76ad417cbcee398e2afbf9b061d1719
(cherry picked from commit 346c52e9)