Update to new OpenID AppAuth-Android version that includes support library v26

[barbeau]

I'm in the process of updating OneBusAway Android to support Android Studio 3.0 and v3 of the Android Gradle plugin.

Android Studio 3.0 now flags mismatched support library versions as an error (although the project still builds and runs ok):

Looking closer, the OpenID AppAuth library is still using support library v24. On OBA Android, if you run:

gradlew -q dependencies onebusaway-android:dependencies --configuration obaGoogleDebugAndroidTestCompileClasspath

You get:

...
\--- com.acrowntest.test:sdk:0.0.37
     +--- com.squareup.picasso:picasso:2.5.2
     +--- com.fasterxml.jackson.core:jackson-annotations:2.6.2 -> 2.7.0
     +--- com.google.android.gms:play-services-gcm:9.4.0
     |    +--- com.google.android.gms:play-services-base:9.4.0 (*)
     |    +--- com.google.android.gms:play-services-basement:9.4.0 (*)
     |    \--- com.google.android.gms:play-services-iid:9.4.0
     |         +--- com.google.android.gms:play-services-base:9.4.0 (*)
     |         \--- com.google.android.gms:play-services-basement:9.4.0 (*)
     +--- com.google.android.gms:play-services-auth:9.4.0
     |    +--- com.google.android.gms:play-services-auth-base:9.4.0
     |    |    +--- com.google.android.gms:play-services-base:9.4.0 (*)
     |    |    \--- com.google.android.gms:play-services-basement:9.4.0 (*)
     |    +--- com.google.android.gms:play-services-base:9.4.0 (*)
     |    \--- com.google.android.gms:play-services-basement:9.4.0 (*)
     +--- net.openid:appauth:0.3.0  // This library is still using support library v24
     |    \--- com.android.support:customtabs:24.0.0
     |         +--- com.android.support:support-annotations:24.0.0 -> 26.1.0
     |         \--- com.android.support:support-v4:24.0.0 -> 26.1.0 (*)
...

The ES SDK is currently using compile 'net.openid:appauth:0.3.0'. Version 0.7.0 is currently available on Maven Central, but this only supports v25 of the support library.

So, we need AppAuth to release a new version that includes support library v26. I've commented on an issue here on the AppAuth project requesting a new release to track this:
openid/AppAuth-Android#298