Set the wmtest targetSDK to 31.

This is to avoid the VerifyApps Google Play Protect popups when running tests.
Tests can set this value to lower if they need it.

Flag: TEST_ONLY
Test: ran a few tests to make sure the popup doesn't show
Fixes: 379261543
Change-Id: I76410173e4229dfd1e019bdb75d6ff65f0891a3b