_FILE_OFFSET_BITS=64 support for asset_manager.h.

Rename off_t based functions to their 64-bit equivalents if we're in
LP32 with a 64-bit off_t.

We can safely do this for all supported API levels now since API 13 is
no longer supported. Clean up all the guards for that API level while
I'm here.

Test: make checkbuild
Bug: https://github.com/android-ndk/ndk/issues/459
Change-Id: I7032e94c7288f87b4895411079216f0ac2cab6bc