[notifications] Check if icon file exists before removing

[cruzach]

Why

We were removing the icon file even if it didn't exists, causing this error:

Config syncing(node:68723) UnhandledPromiseRejectionWarning: Error: ENOENT: no such file or directory, unlink '/Users/charliecruzan/expotests/easmanaged/android/app/src/main/res/drawable-mdpi/notification_icon.png'
    at Object.unlinkSync (fs.js:1210:3)
    at /Users/charliecruzan/expotests/easmanaged/node_modules/expo-notifications/plugin/build/withNotificationsAndroid.js:135:14
    at Array.forEach (<anonymous>)
    at removeNotificationIconImageFiles (/Users/charliecruzan/expotests/easmanaged/node_modules/expo-notifications/plugin/build/withNotificationsAndroid.js:132:38)
    at setNotificationIconAsync (/Users/charliecruzan/expotests/easmanaged/node_modules/expo-notifications/plugin/build/withNotificationsAndroid.js:88:9)

How

checked for file

Test Plan

added test

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.io and README.md).
• This diff will work correctly for expo build (eg: updated @expo/xdl).
• This diff will work correctly for expo prebuild & EAS Build (eg: updated a module plugin).

[expo-bot]

The review previously left here is no longer valid, jump to the latest one 👉 #13539 (review)

[expo-bot]

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Looks like I have nothing to complain about 👏 Keep up the good work! 💪

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