Support per-partition settings for os_version AVB props

The value of the AVB properties are parsed by the device
bootloader, which might expect a different format of the value.
Allowing them to have per-device settings.

The default value of those per-partition os_version is
$(PLATFORM_VERSION_LAST_STABLE), which is usually an integer,
e.g., 12.

Bug: 211364958
Test: build then `avbtool info_image`
Change-Id: I2005df615c0ff35622030baa2477e29d9d20e94f