Update backwards compatibility support for android.test.base

Previously, the plan was for android.test.base to be removed from the
bootclasspath in P, i.e. in the same release as org.apache.http.legacy.
Any apps that targeted < P were to have the android.test.base library
added to their app classpath in order to maintain backwards
compatibility.

Unfortunately, it was not possible to remove android.test.base from P
and instead it is being removed from Q. This update prepares for that
by updating the backwards compatibility support and its tests to add
the android.test.base library to apps that target < Q.

The affected code is only used at runtime when
REMOVE_ATB_FROM_BCP=true.

Bug: 73711752
Test: atest FrameworksCoreTests with and without REMOVE_ATB_FROM_BCP=true
Change-Id: I76b40dad14193cd174114a351b1350c18d647bed