Add use of sentinel in BOOTCLASSPATH entries during preopt.

This change introduces the use of the sentinel string "/./" inside
BOOTCLASSPATH entries to signal the start of the portion of the paths
that should be preserved in optimized dex file dependency lists. It's
a little grotty, but it does serve the purpose of allowing host side
dexopt to do its thing inside a build directory without letting the
host path leading up to that build directory to leak into the results.

This change also makes it an error (instead of just a warning) for
there to be BOOTCLASSPATH entries that are not absolute paths (either
truly absolute or ones with the sentinels as per above).

FWIW, dx uses this sentinel in a similar way.

Change-Id: Ic8d0533d3ee0bd7a1d4d06fcf9232c56f0a60abf