Change screen pinning back to old gesture

This makes the code a bit more complicated, but allows apps to
receive long-press back again and also makes the mode harder
to exit for toddlers.

Test: Without TalkBack enabled, make sure the following works:
- Enter multi-window mode with long pressing recents
- Long press back gets sent to the app.
- Enter screen pinning, make sure you can exit with gesture
Test: With TalkBack enabled, make sure the following works:
- Enter multi-window mode with long pressing recents
- Long press back gets sent to the app.
- Enter screen pinning, make sure you can exit with gesture
Fixes: 324407
Change-Id: Icd7347aa9c60af00fdb0b3991baabb730552d097