Allow functions to get the result associated with a OncePer key without
also specifiying a function. Panics if the key has not already been
set. Also replace the open-coded concurrent map implementation with
the new sync.Map.
Test: m checkbuild
Change-Id: I814fdb1ffffaee8398dc877af146e29638c8a6a8
Add DeviceConfig to store per-device configuration information. Put a
OncePer object inside Config and DeviceConfig, which computes a value
once per key per object to allow build logic to store arbitrary
per-build or per-device computed values.
Change-Id: I1a38b426f29d223ef5e803e0d4d9604500de2fd2