fw: GlobalActions: Always dismiss any existing dialog

The check for "mUiContext == null" is always false after the initial
dialog is created, causing the dismiss logic to never trigger.  All
code uses getUiContext() now, so the check is not necessary.  Remove
it and re-align the create/dismiss logic with AOSP.

Jira: BACON-4069

Change-Id: Ibba7b38a5fdc8410824db6d0a6f726f4a2822c9d