Deterministically report path errors to all callers of dexpreopt.GetGlobalConfig

dexpreopt.GetGlobalConfig uses a OncePer to only parse the config once.
If the config contains missing paths, which can happen in partial
manifest branches, the missing path errors will only be reported on
the first caller to GetGlobalConfig.  Fix the nondeterminism by
wrapping the PathContext and capturing any path errors and reporting
them to all callers to GetGlobalConfig.

Bug: 207813628
Test: tradefed branch builds
Change-Id: I869c3ae49a819b1251b94b73e0487aa594cd5e22