RTSP: add default implementations in NuPlayer for rtsp changes

Change access modifiers and qualify methods with virtual, to
allow extensions in RTSP. Add facility for loading customizations
(Extended classes) dynamically.

List of squashed changes:

rtsp: keep video damaged access unit to enhance fault tolerance
Change-Id: I3b7fb4b098aba5daf149cf36dab7e9380c6d2f69

rtsp: notify the client when receiving BYE message in RTCP
Change-Id: I461960f28610f998b71a6e1322fe79f9856c7a92

rtsp: do not drop early access units of seekable rtsp
Change-Id: I12793dbbf367650e66532195324adb5b5ad8fe85

NuPlayer: Fix ANR while resetting RTSP playback
Change-Id: I3f9efd9fefa66104ad452559ced5ff5218d73a66

RTSP: add RTSP extensions for customization support
Change-Id: I67adeb54b35d1f01911625bb9bad27e94ad0caf0

RTSP: add RTSP extensions for IPV6 support
Change-Id: Iaa67070d1832d56e0569dabfd8327c1998f04493

RTSP: add default implementations in NuPlayer for rtsp changes

Change-Id: I365cce317b4478cd1d12e458b5849ea842062a28