libaudiohal@aidl: Fix MMap position reporting

Fix typos in code to use the right position source.

Limit position resetting to the case when 'createMmapBuffer'
is supported by the HAL.

Bug: 302587331
Test: atest CtsNativeMediaAAudioTestCases
Change-Id: I4a35566bccde84712205f986169dd2df4f8ed9b5