Extract generateUpdatableBcpPackagesRule from updatableBcpPackagesRule

This simplifies the process of moving the updatableBcpPackagesRule to
the platform_bootclasspath by separating the gathering of the list of
updatable modules (which differs between the singleton and the
platform_bootclasspath module) from the gathering of the permitted
packages list and generation of the rule which are generally common.

Bug: 177892522
Test: lunch art_module_arm64
      m out/soong/module_arm64/dex_bootjars/updatable-bcp-packages.txt
      - make sure it is not affected by this change
Change-Id: I3cb64310f618059758a32cfe00d3745d52388e49