Removing the useless note line.

Huge update for Android setup.

Update README.md

upgrade versions of android target, compile sdk and support-core-utils

New line is just important....

Add back the `+` to the line.

Adding common steps for a successful installation.
Adding `Pod 'Firebase` as the required step for Pod installation.
Adding common troubleshooting steps.

delete notification channel method

Separates versioning for firebase-core and firebase-messaging.  These…

separates versioning for firebase-core and firebase-messaging.  These are no longer coupled in recent releases by Google.

Group alert behavior

add channel

Fixed processDebugGoogleServices building error

1. Move the google-services plugin to the bottom to avoid a building time error
2. Error message: app:processDebugGoogleServices FAILED

Fix notification grouping