Update connections data structure in ContextMap to be an ArrayList.

It's currently a HashSet, but the stored type does not override the hashCode method. Objects are only added to the set immediately after being created, so there's not even deduplication based on internal address.

Bug: 327849650
Test: atest com.android.bluetooth.gatt.ContextMapTest
Flag: EXEMPT - no logic change
Change-Id: I1472f5b3065a0920dd8ece6b5c773d9cc314183f