Simplify build conditions in predictors_features.cc

chrome/browser/predictors/predictors_features.cc has a couple of
features that are enabled by default only on android. Centralize the
check for Android in a single place and avoid unnecessary use of the
preprocessor.

Change-Id: Id36f844ca829fe564f693f9fb6d58319c42422f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5403773


Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280170}