Zygote: support native-bridge for app-zygote

1. Disable PreInitializeNativeBridge if native bridge is already
initialized. This happens in apps forked from app-zygote.

2. Pass nullptr app data directory to PreInitializeNativeBridge if app
doesn't have any private storage, which is the case for isolated
app-zygote processes.

Test: CtsExternalServiceTestCases
android.externalservice.cts.ExternalServiceTest
testBindExternalServiceWithZygote
Test: CtsSeccompHostTestCases
android.seccomp.cts.SeccompHostJUnit4DeviceTest
testAppZygoteSyscalls
both for Q.sdk_gphone_x86_arm.armeabi-v7a
Bug: 143143718
Bug: 146904103

Change-Id: I652bb604f30a34826a90b04eb72dde59fb56ae90