Do not rename prebuilt apexes for product packaging

https://r.android.com/2901309 added support for handling installation
rules for multiple prebuilts. One of the implementation details there
was to change LOCAL_MODULE name to the source apex name in the generated
.mk files.

This causes issues in partner builds where the google singed apex
com.google.android.foo gets renamed to com.android.foo in
out/soong/Android-$PRODUCT.mk. If the partner builds does not include
the aosp apex in its PRODUCT_PACAKGES, then the google signed apex gets
elided from packaging.

This CL undoes that change. After this CL, the name of the module as it
appears in Android.bp will match the LOCAL_MODULE name emitted for
product packaging.

Test: in build/soong, go test ./apex
Bug: 355682304
Change-Id: Icadbb75609174cff5326f52baf7d53e901f9f1c0