Provide more fine-grained connection requirements

[rfc2822]

• [x] Normal sync over mobile Internet → covered by NetworkType.CONNECTED
• [ ] Normal sync over WiFi only → covered by NetWorkType.UNMETERED if "Only sync over WiFi" is set
• [x] Normal sync over private WiFi
• [x] Normal sync over public WiFi with login (don't sync until user has logged in)
• [ ] Sync with connection in private LAN without validated Internet
• [ ] Sync with Internet connection where Android processes are blocked by firewall so they can't validate the connection
• [ ] Sync with "always-on" VPN (like Blokada) → currently filtering VPNs, but seems that this isn't a good solution because there are real VPNs without validated Internet
• [ ] Sync with real VPN on top of validated Internet
• [x] #345
  • [x] #385
• [ ] If WiFi is used: limit WiFi to specific SSIDs (the SSID filter could also be used even if all connection types are allowed)

Further info:

• mapping of WorkManager → JobScheduler: https://github.com/aosp-mirror/platform_frameworks_support/blob/a9ac247af2afd4115c3eb6d16c05bc92737d6305/work/workmanager/src/main/java/androidx/work/impl/background/systemjob/SystemJobInfoConverter.java#L134

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Because this is starting to overwhelm me, I tried to create a table for the current state (v4.3.6.1):

SCENARIO	WWW	Server reachable	Account Setting	Checking	sync starting
metered or wifi	yes	yes	-	NetworkType.CONNECTED + INTERNET VALIDATED	yes
specific WiFi SSIDs only	yes	yes	Added SSIDs	INTERNET VALIDATED	yes
public WiFi after login	yes	yes		?	yes
public WiFi before login	no	no	-	?	no
WiFi only	yes	yes	Enabled "Only sync over WiFi"	NetworkType.UNMETERED	yes
blocked validation	yes	yes	Disabled validation requirement	INTERNET	yes
VPN	yes	yes	Enabled "VPN Connectivity" (Ignore VPNs)	INTERNET VALIDATED NOT_VPN	yes
VPN	yes	yes	Disabled "VPN Connectivity" (VPN counts)	INTERNET VALIDATED	yes
VPN	no	no	Enabled "VPN Connectivity" (Ignore VPNs)	INTERNET VALIDATED NOT_VPN	no
VPN	no	no	Disabled "VPN Connectivity" (VPN counts)	INTERNET VALIDATED	yes :( (needed for the below to work)
VPN + blocked validation	yes	yes	Disabled "VPN Connectivity" (VPN counts)	INTERNET VALIDATED	yes (see https://github.com/bitfireAT/davx5/issues/345)
VPN without internet	no	yes	N/A	N/A	N/A
LAN without internet	no	yes	N/A	N/A	N/A

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We should also consider whether potential settings should be account-specific or app-wide.

For VPN we may need to distinguish between "sync with and without VPN" and "sync only over VPN".

[nutpantz]

manual sync should be listed and the preferred action regardless of anything else is an attempted sync expected fail or not. manual sync should not be queued waiting for INTERNET VALIDATED

[rfc2822]

Since WorkManager 2.10, we can now specify a NetworkRequest as a WorkManager constraint. This allows to request specific WiFi SSIDs or network types etc.