Clear pipe after onRestoreFile is called

The BackupAgent can override onRestoreFile and ignore the contents of
the pipe. Currently, the FullBackupEngine assumes that the data is
always consumed and when it's not the pipe buffer gets full and the
restore is blocked indefinitely for the whole device.

Test: atest -v android.app.backup.BackupAgentTest#doRestoreFile_consumesAllBytesInBuffer
Bug: 320633449
Change-Id: Ic48a11a8134786d9427ac0ff1636bef561eb6a2a