Make provider icon sizes consistent

Fixes an issue where the icons under "additional providers"
are not the same size. Also, fixes the unit tests for
CredentialManagerPreferenceController.

https://hsv.googleplex.com/4797855484084224

Test: unit tests and manual on device tests
Bug: 278772478
Change-Id: I88d8694189a7529ccc2d92bf1b74bb5bf2268f46