MAINTAINERS: Remove aaronemassey add GRobertZieba

Aaron Massey (aaronemassey - Google) doesn't have the cycles to remain a
maintainer on the charger subsystem. Robert Zieba (GRobertZieba - Google)
has been participating as a collaborator for the charger subsystem.

Remove aaronemassey as a maintainer of the charger subsystem and add
GRobertZieba as a collaborator.

Signed-off-by: Aaron Massey <aaronmassey@google.com>