Only decrement mRemaining once in read().

In the old code AssetFileDescriptor.AutoCloseInputStream.read()
called super.read(), which calls read(byte[] buffer, int offset, int count),
which decrements mRemaining. Then read() decremented mRemaining again
after super.read() returned.

I'm extending the CTS tests for AssetFileDescriptor.AutoCloseInputStream
to cover this.

Bug: 3240844
Change-Id: I70b945abc773d3ce5c1317dad23a0d797b31f111