Add INVOCATION_TYPE_POWER_BUTTON_LONG_PRESS.

Since this is intended to be used by PhoneWindowManager,
this patch also hoists these internal constants out of
SystemUI and into com.android.internal.app.AssistUtils.

Adds the new UiEvent ASSISTANT_INVOCATION_POWER_LONG_PRESS
for future use by calling code.

This patch also fixes an inconsistency in the existing
constant names (internally to these files).

Fixes: 181601214
Test: atest SystemUITests
Change-Id: Ida860d78ef490e0dc119b37287c6ecdff5b9c0de