fs_mgr: fs_mgr_has_shared_blocks() needs logical blk_device

For logical partitions, the fstab supplied during
fs_mgr_overlayfs_setup() does not have the correct blk_device and
thus check in fs_mgr_has_shared_blocks() inside
fs_mgr_overlayfs_enabled() will fail to provide the correct status.

Call fs_mgr_update_logical_partition() to fix this up.  Side effect
is an API change where fstab can no longer be considered const when
passed into fs_mgr_overlayfs_mount_all() and
fs_mgr_overlayfs_required_devices().

Some additional minor cleanup as well.  Move fs_mgr_rm_all() higher
up as-is in the file as it has no dependencies, which will complicate
future changes.  Add --help to adb-remount-test.sh unit test script,
improve error handling and checking.

Test: adb-remount-test.sh
Bug: 109821005
Bug: 117605276
Change-Id: I548d3797d49661529490d1a0bf96b63e57491704