Update tools/signing/linux-signer-sign-android-apks to check for v3 signatures instead of v1

With the min-sdk version change in the alpha version, apksigner does not include v1 signatures by default.

v2 scheme was introduced in Android 7.0: https://source.android.com/docs/security/features/apksigning#schemes

So we should update our signing script to check for v3 signatures instead of v1.