CEC: Handle <Active Source> arriving before TvInputCallback.onAddInput

Resolves a timing-related bug that fails to process the command
<Active Source> at TV boot up. Checks if TV input required for
the command is ready to accept the request. If not, makes sure
the command is buffered, and processed later when the input is
ready.

Bug: 18896770

Change-Id: Id17e5e8468519b17daf61c962dd718ccc56fb0ea