[PVR] Don't disable PVR clients not in AddonDB

AddonDB is not a reliable source for testing existence of an addon, use
AddonManager instead and tag clients as "known" in PVRDB.
[PVR] Drop unused AddonDB instance member.
[PVR][ADDONS] Drop unneeded PVRAddon special case
[PVR] Add optional param to RegisterClient to get registration status
[PVR] Make sure client addons are disabled first time we see them.
Merge pull request #4761 from t-nelson/alt_incorrect_pvr_addon_disable

Avoid incorrect disabling PVR addons (Alternate)