Merge SampleSidecarImpl and StubSidecar.

Merge SampleSidecarImpl and StubSidecar. We do not plan on developing
Sidecar further and having one class will make it easier to unify with
extensions.

Flag: EXEMPT bugfix
Bug: 347964255
Test: atest CtsWindowManagerJetpackTestCases:SidecarTest
Change-Id: I2bdf9f77aace7d8ce581da4b8f6d30eebe4cf390
4 files changed