Add ActivityManager UidImportanceListener to libandroid

Add UidImportanceListener methods to libandroid for native apex use.

bug: 172942349
bug: 159172726
bug: 146507383

test: updated mediatranscoding's UidPolicy locally to use the new api,
and passed mediatranscodingservice unit tests.

Change-Id: I7ca76f057a83b5bcd634deb2408784d7b93e46fc