fs_mgr: handle more bootconfig parameters

As parameters are moved from kernel cmdline to bootconfig,
fs_mgr needs to be updated to handle the new location.
/proc/bootconfig should be checked first, if not present, then check
/proc/cmdline.

Test: atest CtsFsMgrTestCases
Test: launch_cvd
Test: launch_cvd with 4.19 kernel artifacts that do not support
bootconfig
Test: Both of the above configurations with --num_instances 0 or 4
Test: Both configurations with androidboot.boot_devices or
androidboot.boot_device set
Bug: 173815685

Change-Id: I23fb07a17c25c9459833cb931ced79d5ccc3e42a