Fix memory leak on Accessibility screen

The root cause is that androidx.preference.Preference does not implement
equals and hashCode methods, but it is used as Map key (see bug comment2
for more details). Given that Preference.getParent() can find the
category, we can simplify the data structure from Map to List.

Bug: 388696327
Flag: EXEMPT bugfix
Test: Resume/Pause Accessibility screen 100 times
Change-Id: Ib70acbf2147048730f8a4e8fd66731f9efdadecf