InputDevice warning fixes and make warnigs into Error

use const char * for string literals in InputDevice
Remove unused variable in Keyboard

Bug: 35993509
Test: mm in frameworks/native/libs/input builds without the warning
Change-Id: I11242835eae855386c47e56d6c36e5a2f912a28f
diff --git a/libs/input/Android.bp b/libs/input/Android.bp
index 9485d5b..9294419 100644
--- a/libs/input/Android.bp
+++ b/libs/input/Android.bp
@@ -17,7 +17,11 @@
 cc_library {
     name: "libinput",
     host_supported: true,
-
+    cflags: [
+        "-Wall",
+        "-Wextra",
+        "-Werror",
+    ],
     srcs: [
         "Input.cpp",
         "InputDevice.cpp",