audio policy: update capture policy for HOTWORD source

Restore pre Q behavior for HOTWORD capture for non
Assistant applications. This allows non Assistant privileged
applications to capture audio with source HOTWORD and no visible
foreground UI as long as no other app is capturing. This capture
will be silenced as soon as another app starts capturing and will not
prevent other apps from capturing.

Bug: 140799148
Test: manual OK Google and Now Playing tests
Test: CTS AudioRecordTest, GTS HotwordTest
Change-Id: I9473851e2949e42abc02fb5974be21d351810854