AppZygote: start with the correct instruction set

This allows triggering native-bridge initialization if needed.

Test: CtsExternalServiceTestCases
android.externalservice.cts.ExternalServiceTest
testBindExternalServiceWithZygote
for Q.sdk_gphone_x86_arm.armeabi-v7a
Bug: 143143718

Change-Id: If105c4283879407c4fcee33fa6086772d43609a3