Only create a single MediaScanner instance.

The previous logic resulted in creating a new instance for every
invocation, which was both expensive and resulted in onDetachVolume()
not taking effect.

Bug: 141494460
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I0c155c1ac36bcb72e0ccae53870794d30a8f9342
4 files changed