The nextState() function was not able to understand updates that
are deeper than a single level. Example:
nextState( state, { pagePreviews: { enabled: true } } )
Before, this would replace whatever was in the "pagePreviews"
property with { enabled: true }, but not merge it. In some places
a single level of recursion was done manually because of this.
This can be removed now.
This is done in preparation for splitting the "enabled" flag into
separate ones for each popup type.
Bug: T277639
Change-Id: I35911c18018ba7cd1633a4c882b978656c3fee36
