Reduce and speed-up class def searches.

Use the class linker for descriptor lookups from the compile driver so that
dex caches are populated.
Reduce the scope of functions for scanning class paths to just the class
linker where they are performed.
If we see more than a threshold number of find class def misses on a dex file
lazily compute an index, so that future lookups are constant time (part of the
collection code is taken from
https://android-review.googlesource.com/#/c/103865/3). Note that we take a lazy
approach so that we don't serialize on loading dex files, this avoids the
reason the index was removed in 8b2c0b9a.
Remove an implicit and unnecessary std::string creation for PrintableString.

Single threaded interpret-only dex2oat performance is improved by roughly 10%.

Bug: 16853450

Change-Id: Icf72df76b0a4328f2a24075e81f4ff267b9401f4