Make system_server.zip compatible with prebuilt apex builds

This artifact contains the system server jars that were used in the
build. At ToT, this was disabled when prebuilts were enabled to prevent
version skew between the system server jars built from source, and the
system server jars deapexed from the prebuilt apex that was installed
into the system image.

With this CL, this artifact is now compatible with prebuilts. If a
google prebuilt apex is used, system_server.zip will contain the .jar
deapexed from that prebuilt. e.g. if art _and_ btservices prebuilts are
enabled, then system_server.zip will contain the prebuilt jars for those
apexes, and sources for others.

This implementation leverages the fact that soong copies the _selected_
system server jars to out/soong/system_server_dexjars for dex2oat. Soong
is aware of source/prebuilts, and populates this directory with the
correct system server jar.

One side effect of this change is that affects the directory structure
of the zip. Previously, service-art.jar will be present in the zip file
at apex/com.android.art/java/service-art.jar. After this change
service-art.jar will be present in the top-level directory of the zip

Bug: 308188056
Test: the zip is not bit-identical, but I checked that the file contents are indentical (discussed below)
Test: unzip out/target/product/vsoc_x86_64/system_server.zip.before -d system_server.before
Test: unzip out/target/product/vsoc_x86_64/system_server.zip.before -d system_server.after
Test: find system_server.before/ -type f -printf '%s\t%p\n' | sort -n | cut -f2- | xargs cat | shasum
Test: find system_server.after/ -type f -printf '%s\t%p\n' | sort -n | cut -f2- | xargs cat | shasum
Test: the shasums are identical

Test: in internal, lunch cf_x86_64_phone-next-userdebug (next uses bt
prebuilts)
Test: m nothing
Test: aninja -t path out/target/product/vsoc_x86_64/system_server.zip <vendor_dir_path>/BluetoothGooglePrebuilt/com.google.android.btservices.apks; path exists

Change-Id: I213150b7ee2b012fd649b308f3c72bebf5ca7bf8