Have AssetFileDescriptor.AutoCloseInputStream use pread

Previously two AssetFileDescriptors pointing to the same file could be
using the same underlying file descriptor and reading from both would
provide unexpected results since they would both modify the file offset
in the kernel data structure.

Test: Added testTwoFileDescriptorsWorkIndependently() to
android.content.res.cts.AssetFileDescriptor_AutoCloseInputStreamTest
Test: atest AssetFileDescriptor_AutoCloseInputStreamTest
Fix: 168310122

Change-Id: I6c9f9d1de193992736bf39516ab9364af34a47cd