OMS: extract verifyIdmap from createIdmap

For clarity, split IIdmap2::createIdmap into two separate functions:

  - IIdmap2::verifyIdmap [check if an existing idmap file is OK to use]
  - IIdmap2::createIdmap [unconditionally (re)create an idmap file]

Teach the IdmapManager to call verifyIdmap and to proceed with
createIdmap only if actually needed.

Test: atest OverlayDeviceTests OverlayHostTests
Change-Id: I9f6f1192011fcb094adffeca1eb3f709520bbd24