Use release_avf_microdroid_kernel_version to pick Microdroid kernel

This patch just sets up the directory structure for Microdroid kernel
prebuilts, and does the plumbing in Android.bp

Follow up patches will drop a android15-6.1 microdroid kernel prebuilt
into repo, and switch the trunk_staging to use it.

Once the android15-6.6 is stable to run as guest, we will switch to
that.

Bug: 298011555
Test: builds
Change-Id: I4f56e6c98e24709cf8461bded45b2c8035f48130
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
index 233754a..4aaa793 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -522,10 +522,10 @@
     filename: "microdroid_kernel",
     arch: {
         arm64: {
-            src: ":microdroid_kernel_prebuilts-6.1-arm64",
+            src: ":microdroid_kernel_prebuilt-arm64",
         },
         x86_64: {
-            src: ":microdroid_kernel_prebuilts-6.1-x86_64",
+            src: ":microdroid_kernel_prebuilt-x86_64",
         },
     },
     include_descriptors_from_images: [