Conditionally remove atb from bootclasspath

If REMOVE_ATB_FROM_BCP=true then this adds the
framework-atb-backward-compatibility library to the bootclasspath.

Otherwise, it adds the android.test.base library instead.

Bug: 30188076
Test: build, flash and test manually and with FrameworkCoreTests
Change-Id: I3c15c35724426a9cdeef398c5bc4f5176a8fc752