Allow app usage of AccessibilityInteractionClient#getInstance and clearCache before T

These are needed to have a workaround fix for a11y cache filled with
stale data before android T.

NoNonSdkCheck: set maxTargetSdk to S in UnsupportedAppUsage is not allowed yet.

Bug: b/192110374

Test: it builds fine.
Change-Id: Iaa0f68b2b8e4702f6b638cb3060714a22850af7e