Use consistent mmap/unmap sizes in MemoryIntArray

Ensure mmap/munmap pairings for MemoryIntArray always use the same size
argument, rather than re-querying the ashmem size region before unmap.
This prevents any kind of changes to the underlying fd from bleeding
into the size argument used in the munmap call.

A longer term fix will migrate this class to be implemented purely in
terms of SharedMemory and existing Java primitives.

Bug: 498720726
Test: atest FrameworksUtilTests:android.util.MemoryIntArrayTest
Flag: EXEMPT CVE_FIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:daf86f667f9d8922af019155f8a672cca88c6f71
Merged-In: Id6ff65e9fdbb2d98974f9b78c73f005c6dbf9867
Change-Id: Id6ff65e9fdbb2d98974f9b78c73f005c6dbf9867