[ads] Add fade transition to custom notification ads on Android

[GeetaSarvadnya]

Description

Follow up of #14811- Unable to see ad notification fade in/out transition animation in Android

Steps to reproduce

1. Clean profile 1.26.56
2. Launch brave
3. Enter the cmd --variations-server-url=https://no-thanks.invalid --enable-features="AdNotifications<CommandLineStudy" --force-fieldtrials="*CommandLineStudy/Enabled" --force-fieldtrial-params="CommandLineStudy.Enabled:should_show_custom_notifications/true/ad_notification_timeout/0/ad_notification_fade_duration/2000" in command line sting under QA preferences
4. Relaunch brave
5. Enable rewards and view a custom Ad
6. Custom Ad is shown but there is no fade in/out transition animation

Actual result

Unable to see ad notification fade in/out transition animation in Android

Expected result

Should see the fade in/out animation for custom ad notification

Issue reproduces how often

Easy

Version/Channel Information:

• Can you reproduce this issue with the current Play Store version? NA
• Can you reproduce this issue with the current Play Store Beta version? Yes
• Can you reproduce this issue with the current Play Store Nightly version? Not sure

Device details

• Install type (ARM, x86): ARM
• Device type (Phone, Tablet, Phablet): Android Tablet
• Android version: 10

Brave version

1.26.56

Website problems only

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

Additional information

cc: @brave/legacy_qa @tmancey