[Captioning preferences] Fix wrong color if default color selected

Root cause: When the default captioning color was selected, the opacity became 100% and made the opacity preference is disabled. After changing to non-default color, the opacity preference becomes enabled, but it keeps showing cached opacity in opacity preference and the preview still shows the wrong color.

Solution: Cache the latest opacity if default captioning color was selected to show the correct opacity later.

Bug: 241308551
Test: make RunSettingsRoboTests ROBOTEST_FILTER=com.android.settings.accessibility
Change-Id: I7712fb25d622da62d7fb2d017e33f94ef258941b