Whole static link libasync_safe to libbacktrace.a

When libbacktrace.a is statically lined to somewhere, that library had
to add libasync_safe.a to static_libs because libbacktrace.a has
references to libasync_safe.a. But libbacktace depending on
libasync_safe is an implementation detail of libbacktrace, and therefore
its client shouldn't be affected by it.

Fixing this by doing the whole static link to libasync_safe to
libbacktrace.a so that the former is included in libbacktrace.a

Bug: 149569129
Test: m
Change-Id: If7366a240bc945dda9944fe7c111e10d328165bb

libbacktrace/Android.bp

@@ -89,6 +89,9 @@ cc_library {
         },
         android: {
             static_libs: ["libasync_safe"],
+            static: {
+                whole_static_libs: ["libasync_safe"],
+            },
         },
         vendor: {
             cflags: ["-DNO_LIBDEXFILE_SUPPORT"],