Migrate to Android Testing Support Lib (part 7/N)

InstrumentationTestCase and ActivityInstrumentationTestCase2 are
deprecated.  This CL rewrites tests that rely on those deprecated
classes by using Android Testing Support Library.

Bug: 110805255
Test: verified as follows. No new test failures.
    tapas adb LatinIME LatinIMETests arm64 userdebug && \
    DISABLE_PROGUARD=true make -j LatinIME && \
    adb install -r $OUT/system/app/LatinIME/LatinIME.apk && \
    atest LatinIMETests:com.android.inputmethod.keyboard.internal.MoreKeySpecStringReferenceTests LatinIMETests:com.android.inputmethod.latin.settings.AccountsSettingsFragmentTests
Change-Id: I4fefccaa0c480cfba7142ed36883da2f19b6a5f9