[2nd] Add cluster count API

This is a preparation of the inter character justification

This is a 2nd attempt. The previous attempt was reverted due to
SystemUIGoogleRoboRNGTests failure which assumes the JNI signature has
not been changed. The proper fix is update the Robolectric artifacts
used by SystemUIGoogleRoboRNGTest but it cannot be done in the Android
repo, so add a workaround of calling JNI of old signature on Robolectric
environment.

Bug: 283193133
Test: CtsTextTestCases
Test: minikin_tests
Test: SystemUIGoogleRoboRNGTests
Change-Id: I0eb14f432065d4fc87de4e6e8766ef561f879697