Revert "[Power ML] Change UserActivityPrediction flag to enabled"

This reverts commit 12c3b65c5985b37ae77fdb161e09cc21168dbf26.

Reason for revert: Suspect for failing PowerPolicy browser_tests, https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-chromeos-rel/20473

Original change's description:
> [Power ML] Change UserActivityPrediction flag to enabled
> 
> Bug: 862461
> Change-Id: I624d39293e50754629b3b30990828af73f57f0b3
> Reviewed-on: https://chromium-review.googlesource.com/c/1482281
> Reviewed-by: Dan Erat <derat@chromium.org>
> Reviewed-by: Steven Holte <holte@chromium.org>
> Commit-Queue: Jia Meng <jiameng@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#634856}

TBR=derat@chromium.org,holte@chromium.org,jiameng@chromium.org

Change-Id: Ic6cee677470af06e5ef5de8d362a2978a37291c2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 862461, 935044
Reviewed-on: https://chromium-review.googlesource.com/c/1484821
Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634891}
6 files changed