Use Beta icon for ad notification to distinguish between release and beta ads

[srirambv]

Description

Use Beta icon for ad notification to distinguish between release and beta ads

Steps to Reproduce

1. Install Android-core and release
2. Enable rewards on both
3. Wait for ad notification to show up from beta

Actual result:

Expected result:

Should have beta icon to distinguish ad notifications

Reproduces how often:

Easy

Brave version (brave://version info)

Android-core beta

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome?

Miscellaneous Information:

cc: @anthonypkeane @jamesmudgett @SergeyZhukovsky