has_client: Improve subscribing for notification

To improve PTS testing, let's subscribe to all the CCC before making
operations on the control point.

Bug: 238171211
Test: manualy with PTS
Test: atest BluetoothInstrumentationTests
Tag: #feature
Merged-In: Iebb66edff13edea336de32292fc54f311f72d4ed
Change-Id: Iebb66edff13edea336de32292fc54f311f72d4ed
(cherry picked from commit c9352435)