altosdroid: Fix tab updates on Android 11 after rotate

altosdroid: Fix tab updates on Android 11 after rotate

Android 11 appears to have "optimized" application rotation by
regenerating fragments automatically. This means the tab fragments
aren't getting created by TabsAdapter.getItem, so that code didn't
know about them, which caused it to not know which tab was active so
all of the application state wasn't getting updated in the tabs after
rotation.

Fix this by telling TabsAdapter about fragments that are already
created -- altosdroid hears about them in the registerTab hook.

Signed-off-by: Keith Packard <keithp@keithp.com>