Revert^2 "Use boot image extension for framework libraries."

This reverts commit 3fae7662
Reason: re-applying the change after resolving the problem with
coverage builds (in a related CL).

Use boot image extension for framework libraries.
This patch splits the system boot image in two parts:

  - The ART boot image. This is the primary boot image that is
    included in the ART apex and contains dexpreopted Core Libraries.

  - The framwework boot image extension. It depends on the ART boot
    image and contains framework libraries.

The third "apex" boot image (used in the JIT-zygote experiment)
remains unchanged; it is a monolithic primary boot image that
contains both libcore and framework libraries.

Dexpreopting of APKs now uses the framework boot image extension
(which in turn pulls in the ART boot image as a dependency).

Bug: 146462581
Bug: 119800099

Test: aosp_walleye-userdebug boots.

Change-Id: I06c5ac5fca011fa639ed208735462ab32451df3a