Rebrand the term "rollback" to "revert" for apexd

StagingManager is a client of apexd. Rebrand in apexd requires we make
similar changes in StagingManager.

Bug: 144831762
Test: Builds without error
Change-Id: Idf2104e76a89bd8cd08778ca53129377255031af
Merged-In: Idf2104e76a89bd8cd08778ca53129377255031af