build_image: improvements to right size for dynamic partitions

If partition_reserved_size is 0 or undefined, and
use_dynamic_partition_size is true, we should approach no space
and no free inodes automatically.

Estimate the space and number of inodes required, then do a first
pass build to see how much space actually used, and use those values
to refine the estimate.

Depends on tune2fs to report the characteristics of the filesystem,
so only support for ext filesystems.  In the future if there has to
be a more generic ability, either a tool per a filesystem has to be
found, or we will need root capabilities to mount the filesystem to
acquire the characteristics live from the host system.

Test: manual + python -m unittest test_build_image
Bug: 111302946
Change-Id: I933a388be43516b6de7b5007b296765bd5556fde