libstagefright: fix heap use after free issue

Refactor NuMediaExtractor::setDataSource to move
the common code under a function to reduce code
duplication.
And also, fixing a heap based string dereference
after free in NuMediaExtractor::getName

Bug: 245242273
Test: atest android.mediav2.cts.ExtractorTest
      atest MediaSampleReaderNDKTests:MediaSampleReaderNDKTests.MediaSampleReaderNDKTests
Change-Id: Ie6c663d2df85bee5a2d28a9b709aadf4b6cdd47f
(cherry picked from commit 9a3fb4bf)