Add PackageManager feature strings for EROFS.

This adds two feature strings for EROFS, one for the legacy staging
version present in 4.19 kernels, and another for the official mainline
version present in later kernels.

Bug: 215725871
Test: manual test
Change-Id: Ic36424b578f985ef5a826c3a89c243720fecd0a6
Signed-off-by: David Anderson <dvander@google.com>