Turn MAX_LOGLEVEL into a true constant (and fix self-comparison warning)

This replaces the MAX_LOGLEVEL define with a constexpr variable
in order to fix self-comparison warnings in the logging macros
when compiling with Clang. (Without this change, the log level check
in the logging macros is expanded into something like this:
`if (LINFO <= LINFO)`, which triggers a tautological compare warning.)

Source/Android/jni/MainAndroid.cpp

@@ -402,7 +402,7 @@ Java_org_dolphinemu_dolphinemu_NativeLibrary_GetDefaultGraphicsBackendName(JNIEn
 
 JNIEXPORT jint JNICALL Java_org_dolphinemu_dolphinemu_NativeLibrary_GetMaxLogLevel(JNIEnv*, jclass)
 {
-  return static_cast<jint>(MAX_LOGLEVEL);
+  return static_cast<jint>(Common::Log::MAX_LOGLEVEL);
 }
 
 JNIEXPORT void JNICALL Java_org_dolphinemu_dolphinemu_NativeLibrary_SetProfiling(JNIEnv*, jclass,