Sets mAdded to true only when the #setView is done successfully

ViewRootImpl#mAdded was set to true, but the system crashed and
caused exception thrown and ended the method early. In that case,
the mFirstInputStage was not yet initialized, and caused another NPE
while performing the clean-up.

Bug: 385705687
Test: wm presubmit
Flag: com.android.window.flags.fix_view_root_call_trace
Change-Id: I799f563d637468b00efdc7fb0fce9ce1c28d1ee6