Add vndk tag for libunwindstack which must be a vndk-sp

As a VNDK-SP module, Android.bp must have 'vndk' tag as well as
'vendor_available: true'.

The 'vndk' tag for VNDK-SP module is formated as below:
vndk: {
    enabled: true,
    support_system_process: true,
},

VNDK-SP modules will be installed both in system/lib(64) as normal and
in system/lib(64)/vndk-sp as a vendor variant.

Bug: 64395154
Bug: 63866913
Test: build with BOARD_VNDK_VERSION=current
Merged-In: Idb4757988d1799ffcd3341343fcc4dac34bf7ef3
Change-Id: Idb4757988d1799ffcd3341343fcc4dac34bf7ef3
(cherry picked from commit 14d540a1567994301465ad6c820e6e4973f19c85)
diff --git a/libunwindstack/Android.bp b/libunwindstack/Android.bp
index b971a9e..b4a6cba 100644
--- a/libunwindstack/Android.bp
+++ b/libunwindstack/Android.bp
@@ -35,6 +35,10 @@
 cc_library {
     name: "libunwindstack",
     vendor_available: true,
+    vndk: {
+        enabled: true,
+        support_system_process: true,
+    },
     defaults: ["libunwindstack_flags"],
     export_include_dirs: ["include"],