Fix database log level test on Android.

On Android it's only possible to configure the log level before any operations
have been performed on the object.  This modifies the Unity log level test to
use a pristine database object.

This also updates API documentation with the restriction.

PiperOrigin-RevId: 255409058

Author: smiles

database/src/include/firebase/database.h

@@ -165,6 +165,9 @@ class Database {
 
   /// Set the log verbosity of this object.
   ///
+  /// @note On Android this can only be set before any operations have been
+  /// performed with the object.
+  ///
   /// @param[in] log_level Log level, by default this is set to kLogLevelInfo.
   void set_log_level(LogLevel log_level);