Address API review for ProxyFileDescriptorCallback

The CL revise the documentation for ProxyFileDescriptorCallback.

 * Added explanation to onFsync.
 * Added explanation about offset.
 * Mentioned ErrnoException should contain E constants in OsConstants.

Bug: 35813046
Test: Build succeed
Change-Id: Ied2490b1913445ea8240eb3aaf7c12170ae4e42d
(cherry picked from commit 9555e302)