MotionEvent: Don't update private flags when setting flags from Java

Previously, setting any MotionEvent flags from Java would result in the
private flags (which are only used in cpp) getting cleared.

Ensure private flags are not changed when setting flags from Java.

Bug: 361261924
Test: atest MotionEventTest
Flag: EXEMPT bug fix
Change-Id: Ic9a78db6dbe2e880e321465f797298ae624d6f4d