Fix push foreground listener after re-enabling push service

[MatkovIvan]

• Has CHANGELOG.md been updated?
• Are tests passing locally?
• Are the files formatted correctly?
• Did you add unit tests?
• Did you test your change with either the sample apps that are included in the repository or with a blank app that uses your change?
• Did you check UI tests on the sample app? They are not executed on CI.

Description

1. Start Sasquatch
2. Go to settings
3. Disable push
4. Restart app
5. Go to settings
6. Enable push
7. Stay on settings screen, do not turn screen off and on
8. Push => no dialog

Reason is that mActivity is null in that case for Push class, we need to store/update mActivity reference even when Push is disabled so that it's ready at enabled time.

[coveralls]

Coverage remained the same at 100.0% when pulling 317e722 on fix/push-activity into c545e57 on develop.

[guperrot]

There was a need for the post command, need to find history before I can elaborate.

[guperrot]

Behavior ok, have a comment on unit test.