Avoid duplicated classes for boot dex jars

When a boot classpath java library is directly or indirectly included in
APEXes, multiple variant of the library exist. When running the
hiddenapi tool, we need to eliminate the duplication, otherwise the tool
will complain.

Previously, we simply choose the platform variant of the java library
among the multiple variants. However, if the java library is marked not
available for the platform (i.e. "//apex_available:platform" is not in
the apex_available property), the platform variant does not exist and
thus it is not fed into the hiddenapi tool, which causes missing
references in the tool.

To solve the problem, the platform variant is selected only for the java
libs that are available for the platform. For those libs that are not
available for the platform, we choose one variant of it using a
heuristic; skip testing APEXes and choose com.android.art.release among
the com.android.art.* variants.

Bug: 128708192
Test: m

Change-Id: I33bf297eb3029696ae3504a011486210708fb2c2