Add AppOpsManager.opToDefaultMode() as system API.

PermissionController needs to know the default mode of an app op and
adjust it in case a background permission is fixed to revoked while a
foreground permission is not for a pre-M app, because simply resetting
the app op to its default mode might break the fixed state of the
background permission.

This change also removes AppOpsManager.resetUidMode() since it was
introduced for the same use case in PermissionController but cannot
handle the above case cleanly.

Bug: 110557011
Test: build
Change-Id: I53b1b375381a46b7dbece3cddaac460352d0336d