fix(high contrast text): fix highlight/selection is obscured by high contrast...

fix(high contrast text): fix highlight/selection is obscured by high contrast text background rectangle

Since high contrast text draws a solid rectangle background behind the
text, it covers up the highlights and selections. In this case we draw
over the top of the text with a blend mode that ensures the text stays
high-contrast.

Fix: 323964406
Bug: 186567103
Flag: ACONFIG com.android.graphics.hwui.flags.high_contrast_text_small_text_rect TEAMFOOD
Test: atest core/tests/coretests/src/android/text/LayoutTest.java
Test: manual
1. adb shell setenforce 0 && adb shell setprop persist.device_config.aconfig_flags.accessibility.com.android.graphics.hwui.flags.high_contrast_text_small_text_rect true && adb shell stop && adb shell start
2. Settings -> Accessibility -> Display Size and Text
3. Turn on High Contrast Text
4. Select some text. The selection should be visible

Change-Id: I3ea2835f2a8bdfaf646f140f3290837535080a5c