[Cashtab] Add ecash logo as icon for price notifications

Summary: Price notifications should have a logo. The eCash logo is best. Potentially some ambiguity with the 'eCash received' notifications but imo this is still an improvement on no icon.

Test Plan:
Compare implementation to eCash received toasts. Can change the code to manually trigger one.

Before:

After:

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15918