Fix connection of OUT_REMOTE_SUBMIX device on APM startInput

Fix a regression in AudioPolicyManager::startInput() where
 the OUT_REMOTE_SUBMIX audio device was never made available
 due to the test on the input descriptor, which was already
 active by the time it is tested, as the activity on the
 session has already been updated.

Add methods to AudioInputDescriptor and AudioSessionCollection
 to query the number of active sessions. Use the new method
 to optimize AudioSessionCollection::hasActiveSession()

Bug 26798796

Change-Id: I2de4e47970505e64c4c78a148a80a9f0ec19e821