[Dual Shade] Break the circular calculation of isFullWidthShade in AL.

When Dual Shade is off, isFullWidthShade is determined by the config
value (config_use_split_notification_shade). This value is defined as
`false` by default, but overriden to `true` on large screen devices in
landscape orientation.

In a landscape AL device, `config_use_split_notification_shade` becomes
`true` and causes isFullWidthShade to be `false` which would normally
show Split shade. However, since Split Shade is disabled on AL, the
device shows Dual Shade.

Notice how I wrote "when Dual Shade is off" above. Since that condition
is no longer met due to what happened in the last paragraph,
isFullWidthShade suddenly becomes `true`. This in turn causes it to
revert back to Single Shade, so now the condition "When Dual Shade is
off" is `true` again, and the cycle repeats.

This CL breaks that cycle, and renames `isSplitShadeEnabled` to
`isSplitShadeDisabled` in order to better convey that this config value
doesn't mean that Split Shade would be shown. The associated logic is
also arguably easier to follow as `isSplitShadeDisabled`.

Fix: 436926610
Test: Added unit tests.
Test: Existing unit tests pass.
Flag: com.android.systemui.scene_container
Change-Id: Ice43f55f4cd5e9336c372e33cd4f81dfa7f3e1fb