Enable shrinking for EasterEgg

This reduces the dex size by ~1.5MB (~85%). Note that we intentionally
use a very conservative Proguard config to minimize risk, as this target
isn't as widely used or tested as other system apps. In any case, most
of the reduction comes from trimming transitive deps, as the app layer
is relatively thin.

Test: m + toggle several easter eggs
Bug: 203241149
Change-Id: I5e13e2246053fde2c4ac71e38a28f8de5c6c51c7