Convert `init_vendor` to Android.bp

`init_first_stage` is a dependency of `init_vendor` only when
`BOARD_USES_RECOVERY_AS_BOOT` is false.
Since `BOARD_USES_RECOVERY_AS_BOOT` is already defined in
`build/make/core/android_soong_config_vars.mk` within a
soong_namespace, we can use the `soong_config_module_type` to easily
convert this to Android.bp.

Bug: 347600829
Test: m init_vendor
Change-Id: I1ddcd5fb62983b01e51452c9b7367750e03e7f48