fs_mgr: add fs_mgr_has_shared_blocks and make default if overlayfs

Add fs_mgr_has_shared_blocks in fs_mgr.  Use it as a default decision
to utilize overlayfs if not overridden by the platform.

Test: compile
Bug: 109821005
Change-Id: Ifab22a4c9898966515e268349c24bb4c28a39368