liblp: Indicate usable regions in ResizePartition

- Open up GetFreeRegions() API.
- Add a new argument, free_region_hint, to ResizePartitions(). It
  indicates the regions that extents can be allocated to.
- Expose Interval::Intersect functions. libsnapshot needs the algorithm
  to intersect extents to find out the free regions for COW partitions.

Test: liblp_test

Change-Id: I0c079c0e919aa7b0627eb76a071a7cc2a281d692