Car api review: CarAppContextManager renaming to CarAppFocusManager.

Changes include:
 - rename to CarAppFocusManager
 - allow multiple client listener for listening and setting.
 - change name for set / reset to requestAppFocus, abandonAppFocus
 - rename all APIs to use focus instead of context

Bug: 28886557
Change-Id: I5e5285c104435dbd2eea43c4b0e43ae5ead81218
34 files changed