[google-auth-library-java] Add support for reading GOOGLE_APPLICATION_CREDENTIALS as a property

In Java, we can have "system properties" as well as "environment variables" that keep our map between keys and string values.
We access system properties using the method **System.getProperty** while to access the environment variables we use **System.getenv**.
Different systems may use different approaches to pass configurations. For instance, Amazon Elastibewnsatck passes our variables to **Tomcat** as properties that will have to be accessed using System.getProperty (see link [here](https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/environments-cfg-softwaresettings.html)).

Currently, **DefaultCredentialsProvider** reads GOOGLE_APPLICATION_CREDENTIALS only using System.getenv (apart from using getWellKnownCredentialsFile).
It would be nice if it could also try to read the credentials from properties.

I was unable to send a pull request (permission denied), so here goes the draft code I think would easily fix the issue:
New method for DefaultCredentialsProvider:
```
String getEnvOrProperty(String name) {
String envValue = getEnv(name);
if (envValue == null || envValue.length() == 0) {
envValue = getProperty(name, "");
}
return envValue;
}
```
Change line 135 of DefaultCredentialsProvider#getDefaultCredentialsUnsynchronized from:
`String credentialsPath = getEnv(CREDENTIAL_ENV_VAR);`
to:
`String credentialsPath = getEnvOrProperty(CREDENTIAL_ENV_VAR);`

Another approach would be to modify **GoogleAuthUtils#getWellKnownCredentialsFile** to add a new clause based on getProperty and the default CREDENTIAL_ENV_VAR.