Mitigating cache corruption in ArrayMap

Before calling freeArrays, storing the array reference into a temporary
pointer so that the main pointer doesn't end up corrupting the static
pool of arrays on concurrent access. An earlier change missed a rare
case when removeAt was called for the last element in the map.

Test: atest android.util.ArrayMapTest
atest android.util.cts.ArrayMapTest

Bug: 78898947
Change-Id: I454c5b1600eb0a6c690e746df10f6a0ebcd8aa1d