[PriceTracking] Initialize notification channel with IMPORTANCE_DEFAULT

The notification channel was initialized with IMPORTANCE_LOW which makes
notification silent. This CL raises the importance to DEFAULT which will
make a sound. User can always change the importance level later after
the channel initialization.

Bug: 1286142
Change-Id: Ic68bb0a1edf2804c2646398f187550f9b4719cfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3379325


Reviewed-by: Ayman Almadhoun <ayman@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Zhiyuan Cai <zhiyuancai@chromium.org>
Cr-Commit-Position: refs/heads/main@{#958137}