Show a warning if calendar or contacts storage is deactivated or missing

[sunkup]

Purpose

Sometimes users (mostly on Lineage) disable system apps. We can show a warning in the accounts view and a notification on sync with link to the manual.

Notification at sync seems complex and should be a separate issue: #1253

Short description

Warning in Accounts View

• no need to request permission in manifest, because contact and calendar storage are system apps.
• use packageChangedFlow from BroadCastReceiverFlow to check whether contacts and calendar providers are installed and enabled
• iff missing/disabled: show action cards with warning and link to DAVx5 manual (introduction)

For testing these adb commands are helpful:

adb shell pm disable-user --user 0 com.android.providers.calendar
adb shell pm enable com.android.providers.calendar

adb shell pm uninstall -k --user 0 com.android.providers.calendar
adb shell cmd package install-existing com.android.providers.calendar

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[ArnyminerZ]

Just this detail. I haven't been able to test though, just checked out the code.