Add interface to query the mmap support.

Currently, the mmap support is defined in system property. But the
vendor may set the values wrongly which may cause some CTS tests fail.
These values are best known by the HAL. In that case, add interface to
query MMAP support.

Test: atest AAudioTests
Test: Run OboeTester TEST OUTPUT
Test: Verify that MMAP is showed as enabled if supported
Test: Try toggling MMAP on and off when opening streams
Bug: 193275465
Change-Id: Ida9060b93fa3315031fc451bdea2004ba95207e7