Switch from `count` to `N` for template due to ambiguity with `std::count`

Recent versions of Clang will flag this use of `count` in namespace
`std` as ambiguous due to the existence of `std::count`. To remove this
problem, we can switch to the commonly used `N` for array sizes.

Bug: http://b/155835175
Test: m with aosp_blueline
Change-Id: I983180b6e2e94dadb095f531e13ea415468ee104
diff --git a/keystore/KeyStore.h b/keystore/KeyStore.h
index 0027ec8..7841a80 100644
--- a/keystore/KeyStore.h
+++ b/keystore/KeyStore.h
@@ -62,9 +62,9 @@
 }  // namespace keystore
 
 namespace std {
-template <typename T, size_t count> struct tuple_size<keystore::Devices<T, count>> {
+template <typename T, size_t N> struct tuple_size<keystore::Devices<T, N>> {
   public:
-    static constexpr size_t value = std::tuple_size<std::array<T, count>>::value;
+    static constexpr size_t value = std::tuple_size<std::array<T, N>>::value;
 };
 }  // namespace std