Fix for bug #694405 - iOS Pdf Crash

If an iOS app uses too much memory, the OS asks it to free up some space.
If it doesn't do so in a timely manner, it will get a second warning before
being killed by the OS.

In other platforms, where malloc() return NULL in OOM, the store scavenger
releases memory when mallocs fail. In iOS, mallocs usually never return NULL
because the app is killed before this can happen. Therefore, we need to
initiate a scavenge from the low memory notification instead.

We evict the store to 50% of its current size when a memory warning occurs
when it is in the foreground, and 0% when a memory warning occurs whilst
it is in the background. Having said this, I didn't manage to get a background
warning to occur, presumably because we don't request background execution
Therefore, I think in practice the OS just kills the process. However, this
will be useful if we ever add background execution.
Fix bug #695132: MuPDF Library throwing sporadic exception on Android

Android sometimes calls the 'getSelectedView()' method of an AdapterView.
This can be made to happen more predicatably by enabling the Talkback
accessibility feature.

Remove the UnsupportedOperationException and just return null, as we the
ReaderView does not have the concept of a selected page.