Fix bug in TaskStackBuilder#addParentStack
Fix bug in TaskStackBuilder#addParentStack

Don't assume the initial component has a valid parent specified.

Bug 6464209

Change-Id: I22aa4c0e667ea85dc46ad7320f4a9f52117af520

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base