Add guest OS capability: SecretkeeperProtection

Not all guest OS are capable of interacting with Secretkeeper. Add a
capability "secretkeeper_protection" (which can be extracted from vbmeta
property "com.android.virt.cap"). Add this property to Microdroid
kernel.

pvmfw will have check if the guest OS has this capability & ensures the
rollback_index > 0 if the guest OS has it. Note that this will be
factored in while pvmfw check if updated guest OS should be accepted.

Bug: 291213374
Test: avbtool.py --info microdroid_kernel & check if property is
present.
Test: #payload_with_multiple_capabilities

Change-Id: I99c159d3d65005ec02729b47620ac05ab8d1ec5e
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
index bac93a4..b494cfa 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -418,6 +418,7 @@
     ],
     properties: [
         "rollback_index",
+        "props",
     ],
 }
 
@@ -447,6 +448,12 @@
     soong_config_variables: {
         release_avf_enable_llpvm_changes: {
             rollback_index: 1,
+            props: [
+                {
+                    name: "com.android.virt.cap",
+                    value: "secretkeeper_protection",
+                },
+            ],
         },
     },
 }
@@ -487,6 +494,12 @@
     soong_config_variables: {
         release_avf_enable_llpvm_changes: {
             rollback_index: 1,
+            props: [
+                {
+                    name: "com.android.virt.cap",
+                    value: "secretkeeper_protection",
+                },
+            ],
         },
     },
 }