Use Source_apex_name for LOCAL_MODULE in Soong gen Android.mk

For versioned mainline prebuilts, the version is getting written to
`LOCAL_MODULE` property in the generated Android.mk file, even when they
set Source_apex_name. This CL uses Source_apex_name as the LOCAL_MODULE
so that the versioned prebuilt does not get elided from product
packaging.

Test: m nothing --no-skip-soong-tests
Bug: 405153030
Change-Id: I15a6c6d8a3906cced87771ddde05563209c26bc2