Make graphics.allocator HIDL service optional

This allows the use of the AIDL graphics.allocator AIDL service by
itself.

Bug: 205761012
Test: atest hal_implementation_test
Test: Build and boot cuttlefish without the HIDL service
Change-Id: Ief394181ab2193c56e05c406ca5ba120a988835c
diff --git a/compatibility_matrices/compatibility_matrix.current.xml b/compatibility_matrices/compatibility_matrix.current.xml
index 8b3830a..acee459 100644
--- a/compatibility_matrices/compatibility_matrix.current.xml
+++ b/compatibility_matrices/compatibility_matrix.current.xml
@@ -277,9 +277,9 @@
             <instance>default</instance>
         </interface>
     </hal>
-    <hal format="hidl" optional="false">
+    <hal format="hidl" optional="true">
         <name>android.hardware.graphics.allocator</name>
-        <!-- New, non-Go devices should use 4.0, tested in vts_treble_vintf_vendor_test -->
+        <!-- New, non-Go devices should use 4.0 or the AIDL hal. -->
         <version>2.0</version>
         <version>3.0</version>
         <version>4.0</version>