[pm] smooth transition from install progress to incremental loading progress

Previously, regardless of the downloading progress reported by the client,
right before installation completion, we publish the install progress
as 0.9. For incremental installs, because we will publish the
incremnetal loading progress after the installation completion, we
might see the progress going backward, because the incremental loading
progress is likely less than 0.9.

For a smoother transition, we do not overwrite the client progress.
Instead, during committing, publish the incremental loading progress
while making sure the progress doesn't go backwards. After committing,
the launcher will keep showing the incremental loading progress.

+ Making sure the installer's incremental progress listener is
unregistered after session is committed.

See the demo video for an example. Notice that the progress is changed
during "Installing..." (around 00:35).

BUG: 180040329
Test: manual
Change-Id: I6c30b0dac6758fd103ed6a985ca39b6f8557ba8f