Fix crash when using list of enabled a11y services

AccessibilityManagerService#getEnabledAccessibilityServiceList
had an optimization to always return the same, statically
allocated object. This is almost safe if it's being called
from another process, as Binder will copy it. When called from
the same process, however, it's a lot less safe and seems to
have caused a crash.

I think the optimization was already problematic in Binder
calls as well, though. The method grabs a lock, but I think
it's still possible for another thread to call the method
while Binder is making the copy.

I'm removing the optimization and just allocating a new List
to prevent such crashes.

Bug: 36364829
Test: No repro case, so I just ran a11y CTS and unit tests.
Change-Id: Ib64703a7bbed82c6ca000d8703d23819188b4b9b