Move dex files dependencies (en/de)coding to ClassLoaderContext

Encode the full class loader context in the oat file (rather than just a
list of dex files).

The context encoding matches the format used by dex2oat with the addition
of checksums.

Temporarily assert that at decoding time we are operating on a
PathClassLoader until the checking logic covers all supported cases.

Also, bump the version of the oat file because the format of the classpath
key has changed.

This is a transition step to minimize the size of follow up changes.

Test: m test-art-host
Bug: 38138251
Change-Id: I9ec0cfe092ce1afccb741a36e737896880d5f1d2