Fixes color contrast issue for KeyguardIndicationTextView

Summary of changes:
 1. Use wallpaperTextColor instead of color.White or the current top indication's color
 2. Update indication text color whenever there is any configuration change, this is a superset of changes that includes changes to themes as well
 3. Encapsulate access to the color through get/set methods so that the child classes don't change the color field
 4. Make KeyguardIndicationTextView extend DoubleShadowTextView to add double shadow to improve contrast
 5. Apply the double shadow text style when text is not too dark.

Bug: 349297241
Test: everything builds, verified manually
Flag: com.android.systemui.indication_text_a11y_fix
Change-Id: Ia9ad78c7425631eb5231df56ccbe05c854813165