[EnergyEffect] Add kEnergyEffectInNextbox feature

Add a new feature flag `kEnergyEffectInNextbox` to control the energy
effect in the Contextual Tasks input box (Nextbox). The feature is
enabled by default to work as a killswitch.

This change also exposes the feature state to the frontend as a boolean
`energyEffectEnabled` via the WebUIDataSource in ContextualTasksUI, and
that value is set to a property `energyEffectEnabled_` in
`compsebox.ts` of contextual tasks. This location has been chosen so
that we can easily gate the further energy-effect-related features.

Bug: b:505890048
Link: https://chromium-review.googlesource.com/id/If79d3113ad8943c5cb1d933f53a1dc396a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7790719
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Sho Mizoe <shomizoe@google.com>
Cr-Commit-Position: refs/heads/main@{#1619932}