Fixed CursorWindow signed math for x86 builds.

All tests for our recent CursorWindow changes have been passing for
ARM 64-bit builds, but they weren't executed against 32-bit x86
builds until after merged.

It's not actually safe to use the "off_t" type, so we need to cast
to "int32_t" when doing checks against possible-negative values,
such as in allocRow().

We also add tests that verify negative rows/columns are identified
as invalid positions, which requires that we check the resulting
pointer against both mSlotsEnd and mSlotsStart.

Bug: 169251528, 171276404, 171275409
Test: atest libandroidfw_tests:CursorWindowTest
Test: atest CtsDatabaseTestCases
Change-Id: Iea5f7546850f691e183fbb6e6d0952cd02b00d0f