Explicitly specify `--api-overloaded-method-order source` for metalava

Previously, `--api-overloaded-method-order source` was the default if
that option was not specified but the other change in this topic is
switching it to `--api-overloaded-method-order signature` 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: I00e049e0d991e650ab9940a3d495ed2b6e7480f8