front: Remove superfluous call to audioQuit.

SDL is solely responsible for cleaning itself up. We shouldn't do it
ourselves.
ae-bridge: Fix regression where launch crashed after settings changed.

This commit effectively reverts a regression tracked down to commit
2e4f3c70c477a1ac96745d061f141af2066d8a97

Somehow the order of loading (input-android vs other libraries) and/or
the thread from which they are loaded makes a difference.

The bug can be reproduced by launching emulator, quitting, changing a
user setting (e.g. audio plugin), then relaunching the emulator.  The
app would crash hard due to a segfault.  The crash only occurred when
a user setting was changed; never occurred if the settings didn't
change.
front: Always use framelimiter when not using dummy audio.

Enabling framelimiter (the upstream default) minimizes latency with the
audio-sdl plugin. Disabling the framelimiter is only useful for bench-
marking framerate with dummy audio.

In my testing with older devices, I see no benefit to disabling frame-
limiter as a means of increasing framerate.  My guess is that this
commit won't negatively affect older devices, though it would be nice
to know for sure.
front: Synchronize some calls to core to prevent interleaving.

Not sure if this is ever a problem in real life, but it seems these
should be synchronized just to be safe.