Fix: setTransformationMethod in insert mode doesn't working after ag/23164813

CL ag/23164813 didn't update Editor#setTransformationMethod to use
InsertModeController#setTransformationMethod but called
TextView#setTransformationMethodInternal directly instead. This ends
up triggers TextView#setText and then exit the insert mode due to text
change.

This CL fixed the issue and also refactory the logic of update
transformationMethod into InsertModeController so that it's less error
prone.

Bug: 289953621
Test: atest TextViewHandwritingGestureTest
Change-Id: I8373bf80e3ca9e417eaeee0446c124ba5cc5f5ac