Android Support for RetroAchievements

Modify the RetroAchievements login code in Android to pass in a callback, pop a message if login fails, close the login box if it succeeds.

Android: Convert NetworkHelper to Kotlin

Disables state loading and emulation speeds below 100% when RetroAchievements hardcore mode is enabled.

Core: Let any thread call previously host-thread-only functions

We were previously excluding this folder from Android builds because it
didn't contain any files that were used on Android. However, we now have
an OSD font file that we do want to use on Android, and there's also a
few PNG files that will be needed by the RetroAchievements integration.

In terms of file size, this is what gets added:

OSD font: 48.1 KiB
RetroAchievements graphics: 3.5 KiB
Unused graphics: 116.8 KiB

We're still excluding Sys/Themes/, which is 1.1 MiB and entirely unused.

The core no longer cares which thread is the host thread.

Cleaning up Android's HostThreadLock is left for another PR, in part
because the HostThreadLock in NativeConfig.cpp still serves a purpose,
and in part to make any issues easier to bisect.