Cache VerifiedInputEvent class references

Currently, the variables 'clazz' and 'constructor' are not stored properly.
Cache them to make it consistent with all of the other jni code.

This would also prevent a crash in checkjni for using deleted local
reference (it's never made global in the current code).

Bug: 134977432
Test: atest VerifyInputEventTest (run several times)
Change-Id: Icef5271d915fce49b24d987923942fbdc4b6898b