Specify partition_size argument for vendor_boot partition generation

Follows the similar logic for passing partition_size argument for boot
image generation.

This is the last remaining difference between the soong and make
generated vendor_boot partition, and with this topic the two images are
bit identical.

Test: diff soong and make generated vendor boot partition images
Bug: 379945468
Change-Id: I92b2035a3bc1847de97c7ac6fc370688b94fc2c3