Match taskbar edge padding to nav button end insets

When calculating max number of items to show in a taskbar with overflow,
reserve space for taskbar end/start margins that matches inline nav
buttons end spacing.

Bug: 368119679
Test: None
Flag: com.android.launcher3.taskbar_overflow
Change-Id: I1205663ed4455ae1e7807f8d4edf5aaf2beba7d4