build_image: add support for specifying number of inodes [DO NOT MERGE]

Add support for specifying number of inodes when creating
system, vendor, oem partitions.  These are all read-only
and have no use for extra inodes.  Removing extra inodes
saves a lot of space.

Bug: 32246383
Bug: 33252070
Bug: 33428604
Change-Id: I13f1d4614b64a4abc752c42a1c65d3d151481c21