[sag] Move call handlers to functions

Move the handlers for each of the
fuchsia.power.system/ActivityGovernor methods to functions
instead of doing it inline in the stream reading.

Bug: 462180093

Change-Id: I43276041cc9da2ed593231e4a7a5aebc68a7e133
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1434085
Fuchsia-Auto-Submit: Justin Mattson <jmatt@google.com>
Reviewed-by: Michael Brunson <mbrunson@google.com>
Commit-Queue: Justin Mattson <jmatt@google.com>