Give slightly less useless versions to adb and fastboot.

This will let us see (a) whether the user has a legit build or something they
built themselves and (b) what Android release it corresponds to.

This isn't as useful as showing what Platform Tools release we correspond to,
but I'm planning on doing that as a separate line.

Bug: N/A
Test: adb --version ; fastboot --version
Change-Id: Idca489295e3c6f8571146f95822c08808e36b382