Adding proguard flags to avoid a11y methods getting stripped out.

AccessibilityNodeInfoCompat#obtain and AccessibilityNodeInfoCompat() are
both being stripped out by Proguard since DocsUI is not using these
methods. We are using these methods in tests to test a11y behaviors.

Test: Fixed AccessibilityTests.
Change-Id: I4533a0985794e8f7c36d8f81878196890281264f