Add RootTargetPreparer for IME unit tests

The DumpOnFailure files are generated properly, however the test
instrumentation fails to pull them due to a permission denied error.
This adds the RootTargetPreparer to the test confiugration to allow
reading the files.

Flag: EXEMPT testfix
Bug: 399050749
Bug: 395783236
Test: atest InputMethodServiceTest#testBackButtonClick
Change-Id: Ifc1849b7a21e6db4f4237fa476c2b0d62808c7f6