Remove back-door bootclasspath option from Runtime

The 'bootclasspath' option allowed users of the Runtime to load their
own boot class path DexFiles and pass them directly to the Runtime as
an option. This obscures the fact that the Runtime must take ownership
of the boot class path DexFiles.

This change removes the use of the bootclasspath option by dex2oat and
the common runtime tests. For dex2oat, we use the existing
-Xbootclasspath option instead, and introduce a new
-Xbootclasspath-locations option to override the dex locations for the
loaded boot class path dex files. For the common runtime tests, we
simply use -Xbootclasspath.

Bug: 18809837
Change-Id: Idfcd4885390bf0f3dc350993756dd337220def73