Move aconfig flag NOTIFICATION_MINIMALISM_PROTOTYPE

Move the aconfig flag
NOTIFICATION_MINIMALISM_PROTOTYPE to a new
package that is accessible to the settings app.

Bug: 330387368
Bug: 354047572
Flag: com.android.server.notification.notification_minimalism
Test: atest SystemUITests
Change-Id: Ifd5c1d7f5bdda35b1ea7fb3f882781db16a7d435