Define android.software.credentials.xml as prebuilt_etc

Instead of using PRODUCT_COPY_FILES to copy a source file to install,
define a prebuilt_etc module and add it to the PRODUCT_PACKAGES.
android.software.credentials.xml is to be installed in system for
GSI.

Bug: 345373198
Test: See if android.software.credentials.xml is installed in
      system/etc/permissions
Change-Id: Ib9f030e1e0ff6db55a6e07444943407f9ff1c1df