Split "generation" into added and modified.

The reason we created the "generation" concept was as a replacement
for DATE_ADDED and DATE_MODIFIED, and we realized there is a use-case
for having both values, so this change splits our previous work into
GENERATION_ADDED and GENERATION_MODIFIED to match the other names.

Bug: 147666253, 140248585, 142698119
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I09cc3f52946e91fa3e9aaa14b9fd6309f32712c4