Adds Trace Truth Subjects to FlickerLib 3/4

Allows users to check assertions over a Layers or WindowManager trace
using a fluent interface.
Examples:
  assertThat(layersTrace).fillsRegion(display).forAllEntries();
  assertThat(wmTrace).showsAppWindow("nexuslauncher")
      .then().showsAppWindow("chrome").forAllEntries();
  assertThat(wmTrace).showsAboveAppWindow("NavigationBar")
      .atTheEnd();

Adds custom subjects to GoogleTruth assertions library. Each subject
exposes common assetions for each trace. The subjects can optionally
filter the list of entries in a trace.

Test: atest FlickerLibTest
Bug: b/64532606
Change-Id: I49a45de4a258aad8fc902c98c8b8af27982a83fc