Update documentation for speech APIs.

Makes it clear that the intent and service APIs are likely to stream
audio over the network and notes that impact this has on bandwidth and
battery life.
Also clarifies that the service API should not be used for continuous
recognition as a result. The intent API only runs when the activity is
in the foreground so I didn't mention continuous recognition for it.

Bug: 10674392
Change-Id: Ib5e0c16b3124f4b7d892ef5bd4167f23662d2ca3