initialize mNeedGC

Need to initialize mNeedGC as true to ensure that GC is turned on for
the old idle maintenance service.

Bug: 181079477
Bug: 253187285
Test: run "sm idle-maint run" & check whether GC is triggered
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Change-Id: I7397ae48efbfc9147b4bec8389141dfad8d53a0b