Add support for excluding libraries from class loader contexts

A number of tests in the cts/tests/signature/api-check check for the
accessibility of classes from the android.test.base,
android.test.runner and android.test.mock libraries. Some tests expect
to find the classes other do not. Unfortunately, the tests use
libraries, specifically compatibility-device-util-axt, that depend on
the android.test... libraries which causes Soong to implicitly add
<uses-library> entries to the manifest so that they will be accessible
at runtime. That causes the tests that do not expect to find the
classes to fail.

Bug: 209607558
Test: m nothing
Change-Id: I54c194ab23d5a70df790ece3fe98f2b3d6a1c1f6