Implement Mixins for Templates

Implement Mixin functionalities, which defines part of a template
layout, making it reusable in different layouts. For example, the
HeaderMixin allows setting and getting the header text via the mixin,
which allows for clients which uses multiple different layuots to
simplify their code via something like
     layout.getMixin(HeaderMixin.class).setText("Foobar");

Bug: 34163318
Test: ./gradlew connectedAndroidTest
Change-Id: I4348c8bb5b8e640b49c2be0c79c70aa85cf0ebc0