[Status Bar] Re-name StatusBar.java -> CentralSurfaces.java.

Also add some javadoc to CentralSurfaces.java to make its purpose
slightly clearer.

Bug: 197137564
Test: compiles
Test: atest SystemUITests
Change-Id: Id06e7ba7641d6d2bc276cce57101c9c7236bc758
Merged-In: Id06e7ba7641d6d2bc276cce57101c9c7236bc758
128 files changed