Make support-media-compat an API dependency

The extension cannot be used without also using support-media-compat
(e.g. to instantiate a MediaSessionCompat). So it may as well be an
API dependency.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=221072128

extensions/mediasession/build.gradle

@@ -31,7 +31,7 @@ android {
 
 dependencies {
     implementation project(modulePrefix + 'library-core')
-    implementation 'com.android.support:support-media-compat:' + supportLibraryVersion
+    api 'com.android.support:support-media-compat:' + supportLibraryVersion
 }
 
 ext {