Add exemption for framework-sdkextensions-classpaths boot jar.

As long as we have boot jars defined in make, we should try to keep
global.UpdatableBootJars as a source of truth when it comes to classpath
configuration. However, for testing purposes there is a need to add
java libraries to bootclasspath that aren't present in
PRODUCT_UPDATABLE_BOOT_JARS variable.

Ideally, we would unconditionally add all contents to the config for
test apexes, however, I don't fully understand apex merging to support
it.

Bug: 180105615
Test: atest sdkextensions_classpaths_e2e_tests
Change-Id: I552f93289c27b3713e75dcda1faed909b0bfcef2