Explicitly specify `--api-class-resolution api` for metalava

Previously, `--api-class-resolution api` was the default if that option
was not specified but the other change in this topic is switching it to
`--api-class-resolution api:classpath` to match the behavior in
AndroidX. This change explicitly specifies the option needed by Soong's
use of metalava.

Bug: 285140653
Test: m checkapi
Change-Id: Ief3c7f9dfdfa946f21cb048a579ec90309fb350f