Profile compression: Replace method index with difference with the last method_index

Store the difference with the last method index in the profile rather than
putting the actual method index. Leveraging the common difference values,
compressing the profile will reduce the size further.

Results:

App1
    Without - 74K
    With - 58K
App2
    Without - 54K
    With - 30K
App3
    Without 40K,
    With - 24K

Bug: 36281963

Test: dexlayout_test, profile_assistant_test, \
     profman_test, profile_compilation_info_test

Change-Id: I96bcadac250c08aaf23ade7465c5ac6ba9ea75dc