Revert "Revert "adb: support /oem partition""

This reverts commit 0c481d0c743382ea955a80bf32428bbd87fff45e.

The original build breakage is fixed by (a) building the verity
code for eng builds as well as userdebug builds and (b) moving
the exported remount service functions into a new header file.

Change-Id: Ice0c4f97d4db38ab7eb333c7a6e56bbd11123f5b
8 files changed