ldpi: add blank overlay for space savings

Replaces certain unnecessary assets with tiny replacements
(1x1 transparent png or 1x1 jpg). Useful for space savings.

Note: removed Trebuchet overlay for KitKat.

Change-Id: I1ea8f11bcff8e3a35a3885f911cf91107d5c5429
Disable main ldpi overlay

Keep the (still useful) blank overlay active while we sort out the
real LDPI overlay for KitKat.

Change-Id: Ie7ad3f0c5175bac14bb6add4625808bd2954aa2b