Let self-targeting apps be able access OverlayManager

Self-targeting apps can't get an OverlayManager instance without the
system privilege.  This patch makes sure that the self-targeting
application can get an OverlayManager instance via Context.

```
OverlayManager overlayManager =
         context.getSystemService(OverlayManager.class);
```

In order to make the self-targeting feature compatible in the future,
this patch adds SELF_TARGETING_OVERLAY compatibility feature.

Test: atest \
          OverlayHostTests \
          OverlayDeviceTests \
          SelfTargetingOverlayDeviceTests \
          OverlayRemountedTest \
          FrameworksServicesTests:com.android.server.om \
          CtsContentTestCases:android.content.om.cts \
          idmap2_tests

Bug: 205919743
Change-Id: Ib0b619a2a54e22d31e6c14bf700e421688b6f506