Allow server flag library usable from Framework libraries

Making this library double loadable allows it to be linked to from
multiple Framework libraries linked into a single client APK. Without
this change, configurable flags can only be used inside Framework
services because only this library can only be linked to from one
Framework library.

Bug: 234833109
Test: manual test with 'adb shell device_config' commands
Change-Id: I3fd48db45666d02f642534e9140ec9d6a1ce4f68

libflags/Android.bp

diff --git a/libflags/Android.bp b/libflags/Android.bp
index a11f846..7fb1e13 100644
--- a/libflags/Android.bp
+++ b/libflags/Android.bp
@@ -7,6 +7,7 @@
     srcs: ["server_configurable_flags.cc"],
     host_supported: true,
     vendor_available: true,
+    double_loadable: true,
     shared_libs: [
         "libbase",
         "libcutils",