Clear calling uid in isAppEligibleForBackup() and variant

This method is called by bmgr backupnow. Bmgr pid is not system uid.
When executing the method it fails with permission denied because it
can't bind to the transport. Before binding on-demand we haven't seen
this error because we didn't try to bind in the method, it was always
bound.

Bug: 72215413
Test: adb shell bmgr backupnow -all
Change-Id: I82ad712e893abfec4727d568ab48a49945645eb4