Parse Android .map files for unknown filesystem.

When generating a filesystem during the Android build, we also generate
a text file with .map extension with the list of files and the blocks
in the filesystem they are located.

For filesystems unsupported in delta_generator (like squashfs) we use
this text file to produce efficient delta payloads.

Bug: 28150981
Test: Added unittest for parsing. Generated a delta payload of a squashfs image.

Change-Id: I154e72ac785c6f508290daa901fa7958b446c010