r_submix: Fix get_next_write_timestamp and get_input_buffer_size

Fix the implementation to conform to VTS expectations.

out_get_next_write_timestamp must return "NOT_SUPPORTED" (-ENOSYS)
as it is not supported.

adev_get_input_buffer_size must return a non-zero
buffer size for supported formats, regardless of whether
there are any opened streams at the moment.

Bug: 141847510
Test: atest VtsHalAudioV6_0TargetTest
Change-Id: I322bf8cf42415fddb028c9cd10db0ac82089ce85
1 file changed