Rootcanal: do not starve the reader in unittest

On an M1 the writer thread can write faster than the reader can read.
This results in the reader never exiting the loop, causing the test to
hang.

We now lower the rate of writing to make sure the readers can always
keep up.

Bug: 251187705
Test: Unit tests now succeed on M1
Bug: 245578454
Ignore-AOSP-First: cherry-pick from AOSP
Merged-In: Ia675c3b3b0613b2b00a588a803473a3d997bf0ee
Change-Id: Ia675c3b3b0613b2b00a588a803473a3d997bf0ee
(cherry picked from commit c1a2deb6)