Make platform_bootclasspath a singleton module

This is needed in order to allow it to implement MakeVars so it can
create make variables just like the dexpreopt_bootjars and hiddenapi
singletons currently do.

Bug: 177892522
Bug: 179354495
Test: m nothing
Change-Id: Ida5bf8abeecde531e1f6430151650065445804ac