Don't show global actions if already handled

This double firing of power long press was revealed by a recent change
to the global actions dialog (ag/11247230), where long press on power
will close the dialog. This results in the dialog opening and then
immediately closing when using
`adb shell input keyevent --longpress 26`.

Fixes: 155917751
Test: `adb shell input keyevent --longpress 26` opens global actions
dialog and doesn't immediately close it.

Change-Id: I37df6cadf4b12d2c8e000664f906daf40379bb65