Migrate PageClassification to IntDef.

While the generated java files have enums, we avoid using them for
performance reasons. Making a duplicate IntDef allows us to use
annotations in the code, for marginal type safety.

* Post process java proto generation to create new files for squashed
IntDefs from all enums. For example, OmniboxEventProtos.java gets a
OmniboxEventProtosIntDef.java file.
* Only triggers when build_with_chromium because IntDef is androidx.
* The generated IntDef constants no longer suffix with _VALUE
* Updated all PageClassification call sites to use the annotation.

NO_IFTTT=Just adding an annotation.

Change-Id: If973a8a7d00afbaa9627f85e6ecdd349abf24604
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8226924
Commit-Queue: Sky Malice <skym@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1677531}