Add support for shared libaries in class loader context.

For example:
PCL[a.dex:b.dex]{PCL[s1.dex]#PCL[s2.dex:s3.dex]};DLC[c.dex:d.dex]{DLC[s4.dex]}

Rewrite the class_loader_chain_ to be a chain instead of a vector,
to simplify processing and encoding of contexts.

bug: 111174995
Test: class_loader_context_test, test.py
Change-Id: I7c9f71b67a91b43ae534721b43dc4fdb8e0b6ec4