71ce8bb6f00f4d1cbc1012270d6daefdbda4254d got rid of the need to be the
host or CPU thread when writing to the config.

All settings that we care about from an Android perspective are now
supported by the new config system, so we can remove all the Android code
for the old config system. This should have no impact on users.

Because of the previous commit, this is needed to stop DolphinQt from
forgetting that the user pressed ignore whenever any part of the config
is changed.

This commit also changes the behavior a bit on DolphinQt: "Ignore for
this session" now applies to the current emulation session instead of
the current Dolphin launch. This matches how it already worked on
Android, and is in my opinion better because it means the user won't
lose out on important panic alerts in a game becase they played another
game first that had repeated panic alerts that they wanted to ignore.

For Android, this commit isn't necessary, but it makes the code cleaner.

Android: Call OnConfigChanged when resetting a setting

The only settings that were using LegacyIntSetting are now in the new
config system, so there's no reason to have LegacyIntSetting anymore.

Otherwise the value of the setting won't be updated properly.