Split findAndCopyBootJars into separate find and copy functions

The main difference between the dexpreopt_bootjars singleton and the
platform_bootclasspath singleton module is the way they find the
modules to use. The former searches all modules, the latter adds
dependencies on the modules that they need. This change separates the
finding of the modules from the copying of the boot jars for those
modules to make it easier to move the remaining functionality to
platform_bootclasspath.

This temporarily creates a singleton specific copy of the hidden API
function isModuleInConfiguredList() to select the modules in place of
the logic in the getBootJar() method. There is a slight loss of context
information from the error messages but as these methods will be
removed once the boot image creation has moved this is not an issue.

While switching the isModuleInConfiguredListForSingleton() to use the
SingletonContext the error message was fixed to include the name of
the module with the issue.

Bug: 177892522
Test: m nothing
Change-Id: Iaea906da95d9da5301fb964fc593890f2216d336