oak birch: reset data fresh flag when enabled

This cl resets the data fresh flag when a data source user pref is
enabled. Currently, if a type of data is fetched and disabled, the
data fresh flag still remains true. When the data type is re-enabled,
the view will request a new fetch. If another type of data is fetched
before the enabled type, the data fetch callback will be called since
all data fresh flags are marked true even the enabled data has not
been fetched.

A unit test on view side will be added in a following cl.

Bug: b:332330324
Change-Id: I6c310db3ef6490853403f2dc9be13222ecbdc75e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5410838


Commit-Queue: Xiaodan Zhu <zxdan@chromium.org>
Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#12809274}