SyncAdapter: Hilt error handling

[rfc2822]

After a lot of playing around and testing, I chose the simplest modification to work around the problem. I think it can't be completely avoided that syncs are called (except when we would use a "test account" account type for testing, but then we would test something else that we'd use in the real app).

Short description

• This PR handles the "Hilt: component not initialized" exception in SyncAdapterServices, so that the tests don't fail when such an (unwanted) sync is started.
• Adds a comment to sync_calendars.xml to explain what isAlwaysSyncable really does.
• Adds a comment to the SyncAdapter constructor to explain what autoInitialize really does.
• Requires Android P for instrumented tests (throws error on <P) because the tests use MockK features which are not available for <P.

CC @sunkup