Allow input device config files in product and system_ext.

Treblization requires to locate OEM-specific configs in its own
partition. So the input device config file could be located in
/product/usr or /system_ext/usr.

Bug: 186864233
Test: "dumpsys input" and manual testing.
Change-Id: I72ca5c786ba1848edcef5c6f3ae71ce27bf8bdae
1 file changed