set the properity of lib 'openthread-spi' from 'vendor' to 'vendor_available'

The Thread Network HAL uses the lib 'openthread-spi'. If the properity
is set to `vendor`, there will be compile errors when the Thread
Network HAL uses the lib 'openthread-spi'.

Bug: 277286756
Test: Build and run otbr-agent on the emulator.
Change-Id: I615a14e1d898b6062349eb38054ced91a5106963
diff --git a/Android.bp b/Android.bp
index 66909d3..d28437a 100644
--- a/Android.bp
+++ b/Android.bp
@@ -744,7 +744,7 @@
 
 cc_library_static {
     name: "openthread-spi",
-    vendor: true,
+    vendor_available: true,
     local_include_dirs: [
         "include",
         "src",