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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:b9474405880eb483cfda7c13111f985bf7d0247d)
Merged-In: I5de9a80cbbb601bbdf39009f962fb1175751db28
Change-Id: I5de9a80cbbb601bbdf39009f962fb1175751db28