[Android] Use StringSet in SharedPreferences

[JeroenvdV]

This makes adding notifications much faster by avoiding parsing JSON unnecessarily. Further improvements to the encoding of the SharedPreferences will also improve the speed of cancelling specific notifications.

[Kavantix]

notifications should have unique ids not unique json strings

[JeroenvdV]

I ignored this explicitly, although we can solve this by using a HashMap instead of ArrayList in loadScheduledNotifications.

[Kavantix]

This check implies that we cannot switch back to the old implementation and again to this one, the migration will always happen only once

[JeroenvdV]

This is true, in an attempt to reduce the number of extra queries to Shared Preferences made during normal use.

When switching back to the old implementation, the notifications will be lost. When re-switching to the new implementation, the lost notifications will be restored and any new ones will be lost.

Preventing the initial loss is only possible by additionally saving the alarms the old way, which would defeat the purpose of the change.
Doing a second migration is only possible by always checking for presence of an old data object, which would incur a performance penalty each time.

My suggestion to solve the second migration problem is to make it the responsibility of the app and not the plugin. App developers can ensure notifications are explicitly re-scheduled by the app if such a rare situation occurs.