verity_utils.py is a standalone binary

verity_utils.py gets an entry point and becomes a host binary. This is
to support signing images from the "bootimg" module type. Previously
this was done by directly invoking "avbtool" from the soong module, but
that required people to know the partition_size priori. The partition
size may not be known before actually building the partition image
especially when the partition is not for a physical partition but for a
partition in a composite image.

verity_utils.py, when the partition_size is not given, is capable of
calculating the mininum required partition size based on the size of the
unsigned input image file.

Bug: 180676957
Test: m microdroid_boot-5.10
Change-Id: I7bef292fb141c90899b7bdc0748895f95f964829