Fix NPE from NavbarButtonsViewController on FEATURE_PC devices.

When FEATURE_PC is enabled on a device,
DesktopNavbarButtonsViewController is instantiated. Previously,
DesktopNavbarButtonsViewController saves an internal copy of
mControllers and the mControllers field in NavbarButtonsViewController
is left unset. This was not a problem because mControllers in
NavbarButtonsViewController was not used in any code that was not
overridden by DesktopNavbarButtonsViewController.

ag/17944073 added some code that accesses mControllers in
NavbarButtonsViewController. Even though the code would be skipped for
FEATURE_PC devices (due to the TaskbarUIController check, which is
overridden by DesktopTaskbarUIController) in the end, it still caused an
NPE.

DesktopNavbarButtonsViewController does not require most of the button
updates done in NavbarButtonsViewController. Regardless, this CL
provides a workaround to avoid the NPE by setting mControllers in
NavbarButtonsViewController in FEATURE_PC devices.

Bug: 296623609
Bug: 297260211
Test: Manual; build and launch sdk_gpc emulator. Ensure that no crash
occurs

Change-Id: Ica22359b96fd0a277c575a238abb194d762e81ea