Fix an issue that the FoldStateListener induces a memory leak

FoldStateListener is non-static inner class and keeps reference to
the outer class which is CameraManager. As CameraManager also keeps
context with mContext, memory leak could exist.

Changing FoldStateListener to static inner class, so that the class
won't keep the reference.

Bug: 242264787
Test: Manual test with memory profiling
Change-Id: I77652bd94b50eabc80041e13274b619c0da36601