Stop reading redundant policy definition.

(non-generic) PolicyKey fully determines PolicyDefinitions. This
information is currently stored and loaded twice, under
`policy-key-entry` and also under `policy-definition-entry`, e.g.:

```
<local-policy-entry user-id="0">
  <policy-key-entry policy-identifier="applicationHidden" package-name="com.google.android.GoogleCamera" />
  <policy-state-entry>
    <policy-definition-entry policy-identifier="applicationHidden" package-name="com.google.android.GoogleCamera" />
    <resolved-value-entry value="true" />
```

This is wasteful and allows (in theory) for inconsistencies. This CL
obsoletes the internal `policy-definition-entry` entry tag and
deserializes PolicyDefinition using `policy-key-entry` in the
surrounding `local/global-policy-entry`.

After the flag is fully rolled out we can stop writing the flag.

Bug: 335663055
Test: TH + manual
Flag: android.app.admin.flags.dont_read_policy_definition
Change-Id: Ibf3fe8c75ef9a241d4689f02f18c3f85a21e6b66
3 files changed