Change ClassLinker::dex_caches_ from list to unordered_map.

Move the DexFile* pointer from struct field out to map key.
This makes it easy and fast to find DexCache from DexFile.

Add check that given DexFile* is registered only once.

Test: test.py -b -r --host --64
Test: generated images are identical as before
Change-Id: I84a6d6cbf963af2408abe5bb5e4c99d0ca11df78