Move generateUpdatableBcpPackagesRule to platform_bootclasspath

Changes generateUpdatableBcpPackagesRule to use ModuleContext instead
of SingletonContext and moves the call from dexpreoptBootJar's
GenerateSingletonBuildActions method to platform_bootclasspath's
GenerateAndroidBuildActions.

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: I9741ae1eb9573cafe12dd7a17dc1d8449b224dc8