Improve PIA dim causing UI flash

When triggering intallation from file browser, the PIA will go with
InstallStaging (dim) -> ... -> PackageInstallerActivity (not dim) ->
open ExternalSourcesBlockedDialog (dim), this may make UI looks like
a flash screen. There is another problem here, when entering PIA,
the UI will default shows. If the PIA needs to open a dialog like
unknown external source, there is an user-visible odd animation
transition here.

To improve the problem, we let the InstallStaging without dim. The
alert UI will hide if PIA needs to open a dialog. It will show again
if the PIA needs it.

Bug: 299029375
Test: manual. See go/b299029375_video to see the improved result
Change-Id: I6cb969dc8862c5a36cd95fd578f75479a9078940