mcp: reset MediaControlProfile instance in stop()

When McpService was reused (e.g. stop() and start())
MediaControlProfile was not initialized so GMCS is not created.
It could happen even when McpService is recreated since it is
a static variable.

This CL ensures clear MediaControlProfile instance so that
next time McpService is started, GMCS is created again.

Tag: #feature
Bug: 209606901
Test: atest McpServiceTest
Change-Id: Ibcc6cdad7224e3c363b30510fdfcf228828014f6