Clean up android.hardware.common types

Removed unnecessary 'flags' and 'fdIndex' fields from GrantorDescriptor.
Added additional comments for clarity.

Test: builds
Bug: 142326204
Change-Id: Id818ceb593ba2837d56dbfb3351d2127ec50b7ed

common/aidl/aidl_api/android.hardware.common/current/android/hardware/common/GrantorDescriptor.aidl

@@ -18,8 +18,6 @@
 package android.hardware.common;
 @VintfStability
 parcelable GrantorDescriptor {
-  int flags;
-  int fdIndex;
   int offset;
   long extent;
 }

common/aidl/android/hardware/common/GrantorDescriptor.aidl

@@ -16,10 +16,17 @@
 
 package android.hardware.common;
 
+/*
+ * Included in MQDescriptor, for use with libfmq.
+ */
 @VintfStability
 parcelable GrantorDescriptor {
-    int flags;
-    int fdIndex;
+    /*
+     * The offset of this descriptor in the shared memory in bytes.
+     */
     int offset;
+    /*
+     * The size of this descriptor in bytes.
+     */
     long extent;
 }

common/aidl/android/hardware/common/MQDescriptor.aidl

@@ -18,6 +18,11 @@ package android.hardware.common;
 
 import android.hardware.common.GrantorDescriptor;
 
+/*
+ * For use with libfmq. This is created from an instance of AidlMessageQueue,
+ * and is used to pass information required to create another instance of that
+ * queue for fast communication.
+ */
 @VintfStability
 parcelable MQDescriptor {
     /*