Frameworks: Clean up SharedPreferencesImpl

Clean up in preparation for an implementation change. Add missing
annotations. Rename inner lock to be uniquely named. Use the local
map instead of mMap in the commit logic.

Test: m
Test: m cts && cts-tradefed run commandAndExit cts-dev --module CtsContentTestCases -c android.content.cts.SharedPreferencesTest
Change-Id: Id3a798732c83a4aa6487225e2375ade4985852e2