avb: Add support for sparse image files.

By default the build system uses sparse images for system.img so support
this in avbtool. Part of this code is derived from simg_dump.py in
system/core/libsparse.

BUG=31263798
TEST=New unit tests and unit tests pass.

Change-Id: Iace73c994102a51c59e82ce208ee8438155c50e1
7 files changed