Rename update_current to update_prebuilts.

It's used to update more than just "current" now, and update_prebuilts
seems like a slightly more future-proof name than update_sdk which I also
considered.

Bug: 77525052
Test: N/A.
Change-Id: I3a2f548070806ce75a47c448018f7d2f4c286f11
7 files changed