android: Move update checker to common and make available to Android

[ptyfyre]

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Closes #1591

This PR:

• Moves the QT only UPDATE_CHECKER option to both frontends and automatically enables it for vanilla builds.
• Moves the Update checker related settings from uisettings.h to common settings.h and adds them to Android
• Renames the Theme and color settings to App Settings and adds update checker settings there
• Lastly notifies the user on MainActivity similiar to how QT does it

Also adds code which sets CA certificates which are required for httplib to even work on Android.

Possible TO-DO (Although the PR is still ready for testing / review):

• Not enable the cmake option on debug/relwithdebinfo builds.