soong: Align update boot jar variable with system_server jar

Use apex:jar pair values for updatable boot jars to align with
updatable system_server values.

Test: Compiles & flashed device. Ensured that the bootclasspath & system_server class
paths remain the same.

Change-Id: I1d6dfe3325d9091b7c724458be4e6471f9086666