[SB] Remove fillMaxSize in StatusBarRoot's top-level Box.

Because the status bar mixes Compose views with old android Views, the
measurement of items can be wonky. If a child view doesn't get measured
until Compose's *placement* pass, Compose believes that the child
view's size cannot affect the size of the parent because the parent has
already been measured.

This causes b/433578931, where the stacked mobile icon is ellipsizing
even though it has enough room to show.

During the transition from airplane mode -> non-airplane mode, the
stacked mobile icon changes from having no size to having non-zero size.
But, that size change happens too late. That means the system status
icons container on the right side does not re-measure, so the container
doesn't re-size to be wide enough for the stacked icon. Then, the
stacked mobile icon thinks it doesn't have enough space to render fully
so it renders as an ellipsized dot instead.

Removing the `Box(Modifier.fillMaxSize())` means that the AndroidView
has complete control over the measuring, so the container is correctly
re-measured when the stacked mobile icon changes size.

Note: This is probably fixed with the system_status_icons_in_compose
flag, because then the right side of the status bar is 100% in Compose
and this problem only arises when mixing Compose and non-Compose.

Bug: 433578931
Bug: 406832105
Flag: EXEMPT bug fix
Test: With 2 SIMs that stack: Turn on airplane mode, then turn off
airplane mode and quickly open the shade. Once the two SIMs have
re-connected, close the shade. Verify the stack icon correctly shows
(i.e. no ellipsizing dot is shown)
Test: Smoke test of the status bar layout in landscape+portrait, various
font & display sizes

Change-Id: Ibdc0a61f402bf7b515a333ff1b738533f1b50268