Account for all apps offset during taskbar layout

When laying out its contents, TaskBarView generally lays the icons so
they get centered in the task bar - this is done by calculating the
total size of all icons during layout (this will generally be transient
taskbar icon sizes), and substracting extra margins that get removed
from the divider view. After initial layout, `TaskbarViewController`
adjusts the icon view positions to match expected icon sizes (it
offsets icons horizontally reducing margins between icons to match
intended icon sizes for the taskbar type). Additionally,
`TaskbarViewController` translates all apps button container - this
transformation is asymetric, and causes the task bar contents to become
off-center. To account for this, update taskbar layout to reduce the
total icon size used for centering icons by the amount all apps button
is offset. This makes the taskbar off-center after initial layout, but
by the amount by which all apps button is eventually offset (resulting
in centered taskbar content).

This alone worked for left-to-right UI direction, but not right-to-left.
To fix this, correct the reference point from which
TaskBarViewController offsets icon positions. The offset used to be
calculated by distance of an icon index from half of the icon count -
instead offset needs to be calculated relative to mid-index.
For example, icon at index i needs to be offset by
(mid_index - i) * difference_in_icon_size: ((n-1) / 2 - i) * diff).
Code used to work for LTR UI layout because the list of icons contained
invisible qsb view, which incresed both n and indices by 1, so
(n' / 2 - i') * diff_in_icon_size worked fine, as it evaluated to
((n + 1) / 2 - i - 1) * diff = ((n - 1) / - i) * diff

Bug: 372567501
Test: Verify that taskbar icons are centered when it needs to be, both
with ltr and rtl language UI.
Flag: EXEMPT bugfix

Change-Id: Ic06873cc225a4361d9140d72c055db23f446a1ad