fs_mgr: Add support in fs_mgr_read_fstab_file for /proc/mounts

Generate fstab from /proc/mounts.  Basically like any regular fstab
minus the requirement for the Android fs_mgr_flags.  Added a unit
test to confirm that fs_mgr_read_fstab("/proc/mounts") matches the
setmntent("/proc/mounts")+getmntent() content.

As a result, discovered a bug in parse_flags that would report "ro"
if the "rootcontext=..." flag was referenced.

Test: fs_mgr_unit_test
Bug: 109821005
Change-Id: I635ee979e11ee089b09adbedb5f42b6ba78026de