feat(high contrast text): make high contrast text API public

This is so we can test it in CTS, so we can wait for the setting to take
effect before running the test.

It's also so Compose and other UI frameworks can check the setting and
do their own fixes and treatments of text.

Bug: 186567103
Bug: 326442372
Fix: 330241761
Flag: com.android.graphics.hwui.flags.high_contrast_text_small_text_rect
Test: atest cts/tests/tests/uirendering/src/android/uirendering/cts/testclasses/TextViewHighContrastTextTests.kt

Change-Id: I8dd5663e11c37df8aa86202d3a6f20a312ef69d5