Make openWrite more resilient.

Before this change, mMainOutStream is not null, but any of subsequent 'new'-s fails then failWrite will also fail, crashing the system_server.

Bug: 286046288
Fixes: 286046288
Test: presubmit
Change-Id: I2e7159e8944f75a38a676c3f71da3b6933cf0b2a