Android: Netplay controller mapping

- Switch from ActivityResultContracts.GetContent() to OpenDocument()
  which restores ACTION_OPEN_DOCUMENT behavior, granting write access
  to file descriptors opened in C++ via ContentHandler.openFd().
- Replace unsafe force-unwraps (!!) in EmulationActivity with null checks.

UI for assigning netplay players to gamecube ports and wii remotes.

Android: Add global settings search

Using dual core during netplay seems sufficiently detrimental that its worth showing this warning message and the option to turn dual core off when starting netplay.

To make it extra clear that this is measuring the XFB, not the EFB.

These are available in DolphinQt, so let's make them available in the
Android GUI too.

Android netplay