Add interface to query the mmap support.

Currently, the mmap support is defined in system property. But the
system property only indicates if the mmap is supported or not. It
doesn't indicate on which device mmap is supported. In that case, adding
an interface to query the mmap support can help make things clear.

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: Iac289c1a02e6e2ab4076ab6f2b5045efecad97ac