Add Hide app icon option

[ILoveOpenSourceApplications]

Adds an option to hide icon from launcher.

Resolves #2263, resolves #2266 as well as resolves #2361.

Credits: @rufusin

[ale5000-git]

The icon should be enabled by default.

These two should seems to do the opposite:
CheckIn.HIDE_APP_ICON -> getSettingsBoolean(key, true)
android:enabled="false"

[ale5000-git]

Also in my opinion the variable name seems a bit strange:
private fun updateHideAppIcon(enabled: Boolean)

Personally I would choose
private fun updateHideAppIcon(hide: Boolean)
or
private fun updateShowAppIcon(enabled: Boolean)

[ale5000-git]

The icon should be enabled by default.

android:defaultValue="false"/>

The option in microG settings is disabled so the icon should be enabled by default, but you disable the icon in reality here:
https://github.com/microg/GmsCore/pull/2462/files#diff-d629a5125b2f479f4916b4b963c3ed5bc5b8c8e79951dfb805bc1fa4a7384624R597-R605

        <activity
            android:name="org.microg.gms.ui.MainSettingsActivity"
            android:enabled="false"

Edit: Now that I read better, here you don't disable the icon but the MainSettingsActivity so you actually break the microG settings.
This one mustn't be disabled in any case.

This shouldn't be touched => org.microg.gms.ui.MainSettingsActivity
This can be disabled (but not by default) => org.microg.gms.ui.SettingsActivity

[ale5000-git]

Also in my opinion the variable name seems a bit strange: private fun updateHideAppIcon(enabled: Boolean)

May I ask what the variable does here so that I could understand which one from the below two would make sense?

This is just personal opinion but in this case:
updateHideAppIcon(enabled=true)
when enabled = true, the icon is hidden so not enabled.

instead with this:
updateHideAppIcon(hide=true)
when hide = true, the icon is hidden so not enabled.

So actually it is the same in practice but I think the first is just more confusing but it is still personal preference.

@mar-v-in What do you think?

[ILoveOpenSourceApplications]

@mar-v-in & @ale5000-git, a gentle ping for review.