libvm_payload as a cc_library

Define a symbole file for libvm_payload, and request stubs. Make sure
the clients build against the stubs.

Mark the implementation as not required, to prevent the missing
dependencies check triggering; it is actually present at runtime
inside Microdroid.

Bug: 243512108
Bug: 255920361
Test: atest MicrodroidTests
Test: composd-cmd test-compile
Change-Id: I78c33b6fba706fd0e52435ac13d02a25171862f1
diff --git a/compos/compos_key_helper/Android.bp b/compos/compos_key_helper/Android.bp
index cffa1e3..f8dc783 100644
--- a/compos/compos_key_helper/Android.bp
+++ b/compos/compos_key_helper/Android.bp
@@ -29,7 +29,7 @@
         "libcompos_key",
     ],
     shared_libs: [
-        "libvm_payload",
+        "libvm_payload#current",
         "libbinder_ndk",
     ],
 }