Clean up KeyEvent JNI APIs

Directly return a KeyEvent from KeyEvent_toNative, and change the
KeyEvent parameter from a pointer to a reference in KeyEvent_fromNative.

Bug: 279927189
Bug: 245989146
Test: Presubmit
Change-Id: I9c98700ea4645c6c4cb975fabfa25d1ff959bfc3
8 files changed