fix a sign-compare warning and use -Wall

Test: mma
Bug: 34901468
Change-Id: If68c1540a88ea28987fb7359a0ee01cf45263579
diff --git a/power/1.0/default/Android.bp b/power/1.0/default/Android.bp
index 1a5a897..8bdac73 100644
--- a/power/1.0/default/Android.bp
+++ b/power/1.0/default/Android.bp
@@ -17,6 +17,11 @@
     relative_install_path: "hw",
     srcs: ["Power.cpp"],
 
+    cflags: [
+        "-Wall",
+        "-Werror",
+    ],
+
     shared_libs: [
         "liblog",
         "libhardware",
@@ -35,6 +40,11 @@
     init_rc: ["android.hardware.power@1.0-service.rc"],
     srcs: ["service.cpp"],
 
+    cflags: [
+        "-Wall",
+        "-Werror",
+    ],
+
     shared_libs: [
         "liblog",
         "libdl",