Restrict the area for the input to unsatsh the taskbar

The input to unstash the taskbar should only be 48dp more than
taskbar_stashed_handle_width or 316dp for wich I created a new
variable.

Bug: 204166104
Test: Manually stashing and unstashing the taskbar.
Change-Id: I94e2e289fcd1169ed0e38a0c45abca6c0ae5c502
2 files changed