aaudio: legacy reports EXCLUSIVE when not

Legacy streams could return EXCLUSIVE from getSharingMode()
if requested by caller.  But that is not supported.
To avoid misleading developers, we force it to SHARED mode.

Bug: 63355841
Test: setprop aaudio.mmap_exclusive_policy 2 && write_sine -x -pn
Change-Id: I91c82b63293752c6aa2c4933cd2bfe8c6d41a5d3