Switch over from libbrillo flags to gflags

libbrillo is a library from chromeos. It is under-maintained in aosp.
Switch over to gflags.

Test: th
Change-Id: Icc6e74cb1292b39a53b263673874951f490304cf
diff --git a/Android.bp b/Android.bp
index 833d766..c321de8 100644
--- a/Android.bp
+++ b/Android.bp
@@ -558,7 +558,10 @@
         "libupdate_engine_android_exports",
     ],
 
-    static_libs: ["libupdate_engine_android"],
+    static_libs: [
+        "libupdate_engine_android",
+        "libgflags",
+    ],
     required: [
         "cacerts_google",
         "otacerts",
@@ -858,6 +861,7 @@
         "libavb_host_sysdeps",
         "libpayload_consumer",
         "libpayload_generator",
+        "libgflags",
     ],
 
     srcs: ["payload_generator/generate_delta_main.cc"],
@@ -875,6 +879,7 @@
     static_libs: [
         "libpayload_consumer",
         "libpayload_generator",
+        "libgflags",
     ],
 
     srcs: ["payload_generator/generate_delta_main.cc"],