Add DIRTY_RETAIN constant

Previously the DIRTY flag can only be automatically or explicitly set.
There are no option to express "This operation does not change the DIRTY
flag". This causes issue when the voicemail source is downloading
changes from the server instead of uploading and would clobber the flag.

After this CL, if DIRTY is updated with DIRTY_RETAIN, the flag will not
be updated for the operation.

Change-Id: I4ed430865c28c824cbd13ccf884091637b106d8a
Fixes: 64371667
Test: CtsProviderTestCases#VoicemailProviderTest