Clear BOOTCLASSPATH for APP AOT compilation.

Let dex2oat pick up the boot classpath from the boot image,
excluding the updatable module components. This allows
updating those modules without recompiling apps.

Test: Manually check bootclasspath entry in an oat file
      compiled on device.
Bug: 122937705
Change-Id: I353f3862c9ece046a36010aa121d1f88ec9be78f