Audio : Fix a few minor issues with stream switcher class

1. Constructor should forward variable arguments
2. mContext initialization was missing
3. Add nullptr check for addEffect and removeEffect methods

Bug: 264712385
Bug: 286914845
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I4037292bf497be186b26508bd16846886fa4fe55
2 files changed