Add ClasspathElement support

The configuration of the bootclasspath, e.g. PRODUCT_BOOT_JARS,
includes libraries that are standalone and libraries that are part of
an APEX. All libraries that are part of an APEX must also be part of a
bootclasspath_fragment. Currently, the libraries and fragments are
handled separately but that is limiting as it does not make it easy to
process all the libraries while treating those from fragments
differently to standalone libraries. That is needed for hidden API
processing as it needs to use classesJars from standalone libraries
but pre-generated flag files from fragments.

This change adds support for ClasspathElements which is represents the
classpath as a whole while differentiating between standalone libraries
and fragments.

Bug: 177892522
Test: m nothing
Change-Id: I7a4adc67164a46079eb8ec0a17fc755044b4949d