Stage 'android' package in PerformUnifiedRestoreTask

Bug: 145126096
Test: atest KeyValueRestoreExclusionHostSideTest
      atest PerformUnifiedRestoreTaskTest

In a KV restore after getting data from the transport, we save it into a
stage file. Then we go through the keys and do filtering: skip the keys
that should be excluded and extract the widget data into a separate
file. The rest of the data is wirtten into the file where the app's
backup agent will read it from.

However, this process is skipped for 'android' package. It was done as
an optimization before the ability to exclude keys from restore was
introduced: as 'android' backup data doesn't contain any widget info.
However, now we need to process 'android' package as well because it can
contain keys to be excluded.

Change-Id: I612f8cc9c6903c9bd257762360dadb81ed12d106