fix(HCT): shows HCT background for invisible text

Background rendering is skipped for spans with an alpha value of 0.

Bug: 389137049
Flag: com.android.graphics.hwui.flags.high_contrast_text_small_text_rect
Test: atest core/tests/coretests/src/android/text/LayoutTest.java
Change-Id: I5e9a08653c10f6688a27d0ac9943e0f3c5413d86