Optimize GraphemeClusterSegmentFinder performance

This CL utilized minikin:isGraphemeBreak to compute all grapheme break indices when creating GraphemeClusterSegmentFinder. This makes all methods (such as #nextStartBoundary) of GraphemeClusterSegmentFinder O(1) instead of O(n).

Bug: 271004887
Test: atest TextViewHandwritingGestureTest
Change-Id: I61c1f2c45123a584456b4860b72cd4fdc84c5f1c