prebuilt_apex created ApexInfo must not include prebuilt_ prefix

As part of the work to modularize the hiddenAPI processing the
generation of the monolithic hidden API index file needs to be moved
to the platform_bootclasspath module type. Doing that broke the
TestBootDexJarsFromSourcesAndPrebuilts tests which checks the inputs to
the rule that creates that file. Fixing that required added a
platform_bootclasspath module to the test fixture for those tests which
highlighted an issue with the prebuilt_apex module.

Previously, when the prebuilt_apex created apex variants it would use
its own name as the apex variant name, even when that name included the
prebuilt_ prefix. That broke the platform_bootclasspath logic as it was
looking for apex variants for "myapex" but the only ones available were
"prebuilt_myapex".

This change ensures that it always uses the unprefixed name and fixes
the TestNoUpdatableJarsInBootImage to match. This also adds some
improved error reporting in platform_bootclasspath which helped debug
this problem.

Bug: 177892522
Test: m nothing
Change-Id: I3e88b5cec767f77dcc0e94b3ae38b499d07eadf0