Introduce VDEX file, use it for DEX files

This patch introduces a new output file called VDEX. In the future,
VDEX files will store pre-validated DEX files which do not need to be
re-extracted and re-verified when recompiling, e.g. due to new
profiling information or after a system update.

With this CL, the OatWriter writes DEX files into the VDEX and the
rest of its output into OAT. The OatFile class and related classes
are updated to load the VDEX at runtime and mmap the DEX file section
from it. Patchoat creates symlinks to the source VDEX files in the
target directory or copies the files if passed in as file descriptors.

The feature can be disabled by setting the environment variable
ART_ENABLE_VDEX to false.

Test: m test-art-host
Bug: 30937355
Change-Id: I54dcaececf6814c258c80524ec15e2e2ef69c8dd