I was able to get the token by using https://login.microsoftonline.com//oauth2/token. But not able to authorize the app service with this bearer token.
As juunas answered, your token may does not match the AAD provider you configured on Azure Portal. Details you could follow here to check your configuration. Moreover, you could use https://jwt.io/ to decode your access_token and validate the related properties (e.g. the aud should be the clientId you configured on Azure Portal,etc.).
As App Service Authentication / Authorization (EasyAuth) states as follows:
Users who interact with your application through a web browser will have a cookie set so that they can remain authenticated as they browse your application. For other client types, such as mobile, a JSON web token (JWT), which should be presented in the X-ZUMO-AUTH header, will be issued to the client. The Mobile Apps client SDKs will handle this for you. Alternatively, an Azure Active Directory identity token or access token may be directly included in the Authorization header as a bearer token.
For Azure Web App or Azure Mobile App, you could just access your endpoint as follows:
https://{your-app-name}.azurewebsites.net/api/values
Header: Authorization:Bearer {the id_token or access_token of AAD}
Or
https://{your-app-name}.azurewebsites.net/api/values
Header: x-zumo-auth:{authenticationToken}
Moreover, if you retrieve the access_token in your mobile app, you could also use it to retrieve the authenticationToken and use the authenticationToken for communicating with the backend endpoint.
POST https://{your-app-name}.azurewebsites.net/.auth/login/{provider-name,for your scenario, it would be AAD}
Body: {"access_token":"<your-access-token>"}
For your mobile client, you could use the client for Azure Mobile Apps, details you could follow here. Also, you could follow Authenticate users to understand the client-flow and server-flow authentication for App Service Authentication.