Implement persisting pip location

Instead of the pip window being reopened in the default bottom right location each time, it opens in the last (regular) pip window location set by the user. Should persist across reboots as well.

Test: atest TvPipGravityTest
Test: manual: created a test app that can go into regular and expanded pip. Checked that pip position is preserved across pip window close / reboot. Checked that corresponding position is correct when switching between regular / expanded mode across pip window close / reboot.
Bug: 356876580
Flag: EXEMPT BUGFIX
Change-Id: I7e306e707ae8035af83043158eb9e15a7b634dfc