init: Handle properties in the background of calling fs_mgr

It's been a long standing problem that init calls fs_mgr functions
synchronously and therefore stops handling properties, which causes
deadlocks if either fs_mgr, or vdc, or vold attempt to set
properties.

Previous work, b/21904461, shows that there is a large performance
penalty for adding any amount of locking to properties, so moving
property service into its own thread generically is not a viable
option.  However, we can be sure that init is not setting properties
while the fs_mgr functions are running, so we can poll the property
socket in a thread while we call these functions.

The other alternative would have been to separate the fs_mgr functions
into smaller pieces and revisit the main init loop between each
piece.  Unfortunately, this would be difficult, since
fs_mgr_mount_all() calls out to different processes via logwrapper,
which synchronously polls on a logging FD from the child, among other
complexities that would make this strategy much more difficult than it
would be worth.

Bug: 21904461
Test: device boots, including when setting property in
      fs_mgr_mount_all()

Change-Id: Ib0b7123024035884f9d90f9b489c1e2f5a2e1707