Align action_buttons at bottom of OverviewActionsView

- http://ag/q/topic:%22b/329255757%22 caused a regression that in landscape, action_buttons shifted up when ShareTarget become visible, because OverviewActionsView become taller when this happens, and the topMargin is no longer valid to put actions_button at the position we want
- Bring back layout_gravity="bottom" solves the issue, as bottomMarign is always correct

Fix: 331517987
Test: manual as ShareTarget requires AiAi interaction and cannot easily be mocked
Flag: None
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:6431d6e87910bc7c7767c40d42f51961dc462134)
Merged-In: Iadcdf5731485f42f9586703e34e55ed9b4121bf9
Change-Id: Iadcdf5731485f42f9586703e34e55ed9b4121bf9