Android.bp: Add comment about APEX stub dependency for MimeMap.

This addresses a review comment on https://r.android.com/1172924

Test: Treehugger
Change-Id: Ic2106aebf707c9d347f06d5a0dab18614e0ad5a3

Android.bp

@@ -426,6 +426,8 @@ java_library {
         "services-platform-compat-config",
     ],
     static_libs: [
+        // If MimeMap ever becomes its own APEX, then this dependency would need to be removed
+        // in favor of an API stubs dependency in java_library "framework" below.
         "mimemap",
     ],
     // For backwards compatibility.