Update mPosition when creating a new InsetsSourceControl

This should be part of commit: ac6cde62
that InsetsSourceProvider.mPosition should always be the same as
InsetsSourceControl.mSurfacePosition except during mHasPendingPosition.

Fix: 370738846
Flag: EXEMPT bugfix
Test: Click on the IME switch button and see if the IME position is
      correct.
Test: atest InsetsSourceProviderTest
Change-Id: I5de9a80cbbb601bbdf39009f962fb1175751db28