Be more accepting of dex2oat errors.

- Narrow down the scope of caught exceptions. StaleDexCacheError has been
  removed (nobody was throwing it) and we shouldn't be catching the
  general "Exception".
- Make try {} catch blocks for IOExceptions more fine-grained, only
  surrounding the code that throws them.

And finally, don't return DEX_OPT_FAILED if installd couldn't run
dex2oat successfully. We return DEX_OPT_SKIPPED instead.

bug: 20888973
Change-Id: I25701e16157f42981ca767e343545cfe1d9cfbb2