adb: move the FgThread details around setAdbEnabled

This change removes copypasta around setAdbEnables
which will help in the next change.

Bug: 402444824
Flag: EXEMPT cleanup
Test: presubmit
Change-Id: I605ce7195d874c67b4764b45cf6adcde9cc6c338
Signed-off-by: Roman Kiryanov <rkir@google.com>