Handle edge case where overflow icon is overflowing

In certain situations, space in taskbar may be restricted so even the
overflow icon for running apps is technically overflowing available
space (i.e. not fully within intended margins). In such cases the
overflow icon was not rendered, as the size of the overflown icons was
larger than the number of running app icons, so the list of icons for
overflow icon was never fully initialized.
Handle this case by capping the number of items to add to overflow icon
to the number of available running apps.

Bug: 379774843
Test: Enable three button navigation, increase the display scaling (to
effectively reduce available space), and enter desktop session in
portrait orientation. Verify the taskbar overflow icon shows up.
Flag: com.android.launcher3.taskbar_overflow

Change-Id: I06371637d1b01e99eaf30aec98ae0920aa248652