Simplify the flow in RegExpFilter

Clean up the flow in RegExpFilter to make it simpler and consistent with
ChainingRegExpFilter.

Change-Id: If1227e43c2138bc854f56434750657c48de2d20a
Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Merge "Simplify the flow in RegExpFilter"