Push mapping from file extension to GID.

An upcoming sdcardfs change will offer to automatically change the
GID of files on external storage based on the extension, but we need
to tell the kernel the mapping that we'd like to see.

This change pushes the current mappings used by installd when kernel
support is detected.

Test: builds, boots, works with old/new kernels
Bug: 34733702
Change-Id: I4a13b17f5a8e080a9bea51b01aa9f1cbf575e0ab