roots: volume_for_path() parses and tries prefixes.

Commit cc323958f99e40fea06c511656c69c0b2e2d47f7 in system/core has
changed fs_mgr_get_entry_for_mount_point() to do an exact match only,
which breaks the behavior in volume_for_path().

This CL changes the volume_for_path() implementation to parse and pass
prefixes locally. For a given path like "/cache/recovery/last_log", it
will in turn attempt the prefixes of "/cache/recovery/last_log",
"/cache/recovery", "/cache", "/" and return the first hit.

Bug: 63912287
Test: Build and boot into recovery image on bullhead. 'View recovery
      logs' works.
Change-Id: Ic8635b0939649dd5cc9ca501ebc3a2d1fbf5849d