Allow system_server to read vendor_file

Input device configuration files .idc, .kl that are placed in /vendor
are currently not accessible.
Allow the read access here.

Bug: 112880217
Test: move .idc and .kl files from /system to /vendor, then observe
logcat. With this patch, avc denials disappear.

Change-Id: I72ad62b9adf415f787565adced73fd8aaff38832