Refactor return values of performDexOpt

PackageDexOptimizer.performDexOpt would return DEX_OPT_PERFORMED if
dexopt succeeded on the package and DEX_OPT_SKIPPED otherwise, even
if dexopt failed. This patch fixes that and cleans up the code.

PackageManagerService.performDexOpt* would return true only if
PackageDexOptimizer.performDexOpt returned DEX_OPT_PERFORMED.
Consequently, it would return false when dexopt was not needed. This
patch refactors the code to return true unless PackageDexOptimizer
returns DEX_OPT_FAILED and documents the behaviour.

Bug: 28082762
Change-Id: Ica73e67ab02025ef5619746bb8c465c96b72846b