support different boot partition sizes in aosp_arm64

The different boot images in aosp_arm64 have different partition size
requirements. Add support for defining a partition size associated with
each boot image variant.

This support is limited to aosp_arm64 currently, which is built with
recovery-as-boot.

Bug: 156036850
Bug: 155049180
Change-Id: I73b70a4d52c2662c3015899632bb8a2b424108d5