Set Android T as default target for APK signing key rotation am: 6ad4c757ab

Original change: https://googleplex-android-review.googlesource.com/c/platform/tools/apksig/+/18084651

Change-Id: I6ea196c4a438e1583dec238c7fdb6b36e3a3b6b0
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>