[RFCLAT#14] replace "libcutils" with "libcutils_headers"

replacing "libcutils" with "libcutils_headers" can save the apex size
~356KiB (libbase.so + libcutils.so). clatd only needs the definition
AID_CLAT in the libcutils exported headers. It would be more
appropriate for using libcutils_headers.

before:
$ deapexer extract out/target/product/raven/system/apex/com.google.android.tethering.apex apex_with_clat_uses_libcutils
$ ls -all out/target/product/raven/system/apex/com.google.android.tethering.apex
-rw-r--r-- 1 nuccachen primarygroup 4083712 Jan  3 21:51 out/target/product/raven/system/apex/com.google.android.tethering.apex

after:
$ deapexer extract out/target/product/raven/system/apex/com.google.android.tethering.apex apex_with_clat_uses_libcutils_headers
$ ls -all out/target/product/raven/system/apex/com.google.android.tethering.apex
-rw-r--r-- 1 nuccachen primarygroup 3719168 Jan  5 18:40 out/target/product/raven/system/apex/com.google.android.tethering.apex

Both libbase.so and libcutils.so are removed from the apex
which is using clatd with libcutils_headers:
$ diff -r apex_with_clat_uses_libcutils apex_with_clat_uses_libcutils_headers
Only in apex_with_clat_uses_libcutils/lib64: libbase.so (263.4 kB)
Only in apex_with_clat_uses_libcutils/lib64: libcutils.so (91.0 kB)

Bug: 212345928
Test: atest clatd_test
Change-Id: Ia286c1d50be758685e92a6e86faa57b675e9cc54

Android.bp

@@ -53,6 +53,10 @@ filegroup {
 cc_binary {
     name: "clatd",
     defaults: ["clatd_defaults"],
+    // TODO: remove once drop_root_and_caps() is removed.
+    header_libs: [
+        "libcutils_headers",  // for AID_CLAT
+    ],
     srcs: [
         ":clatd_common",
         "main.c"
@@ -61,7 +65,6 @@ cc_binary {
         "libip_checksum",
     ],
     shared_libs: [
-        "libcutils",
         "liblog",
     ],