Update fix_navbar_deadzone to be bugfix

In [1] we missed marking the flag as a bug fix. Since a flag can't be
repurposed, we are renaming it.

[1]: Ib4050a4928427c8746fa5a9737b889aca8c664e5

Bug: 364856257
Test: atest CtsInputMethodTestCases
Flag: android.view.inputmethod.fix_navbar_dead_zone

Change-Id: I9a3595f60fd303e319eaa627d009416d0a8e7084