Move reference WM Sidecar impl to system_ext

Placing the reference implementation in /vendor partition tirggers
UnofficialApisUsageTest failures due to hidden API use in the sample.
Moving it to system_ext should resolve this issue.

Bug: 167214265
Test: com.android.gts.api.UnofficialApisUsageTest#testNonApiReferences
Change-Id: I628fd0e00558208a3af26ecf02b82a3cedad8011
2 files changed