nativebridge: Support app-zygote

1. Allow initialization without app data directory, since isolated
processes don't have any external storage
2. Skip initialization if it was already called. This happens for
an app forked from an app-zygote

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: I08b51504ccba11dd0e5c03ca7ac728d38e7cde9c
2 files changed