Remove Droid Sans Fallback from all builds

Droid Sans Fallback is now completely replaced by Noto Sans CJK. The
font files are kept in the tree, since the SDK still needs them.

Also, Android.mk is updated to reflect the fact that Noto Color Emoji
is now included in SMALLER_FONT_FOOTPRINT, so fontchain_lint checks
for emoji consistency in that footprint too.

Bug: 31854008
Test: make fontchain_lint
Change-Id: I4f57114a48ecfaee87b49f01f7ef4fa5cd0b19fb