Rename Os.sendfile() parameter inOffset -> offset.

The name was misleading because this parameter is used for both input
and output to/from the syscall, not just input: According to the POSIX
API documentation [1],

 If offset is not NULL, then it points to a variable holding the file
 offset from which sendfile() will start reading data from in_fd.
 When sendfile() returns, this variable will be set to the offset of
 the byte following the last byte that was read.

Further, the corresponding parameter in the POSIX API is called "offset".

This CL touches neither the native implementation in libcore_io_Linux.cpp
nor OsTest.java because that code already uses appropriate names.

[1] http://man7.org/linux/man-pages/man2/sendfile.2.html

Test: Treehugger
Bug: 68927116

Change-Id: Ic0f93fe5e0f9e26282fc583a95ad3ba0cf9a4066