Restoring starting capturing existing activity.

ag/24138143 made the rule to produce empty captures in many cases.

ag/24138143 aimed to fix a leak that was caused by a local var
alreadyOpenActivity still referring the activity when the leak check
executes.

Fixing that by moving the variable to a method startCapturingExistingActivity.

Bug: 291638593
Test: local, presubmit
Flag: N/A
Change-Id: I281202488c6c85e2e2c5b5b3300e26d808167104