Update apidiff source files to use new api location.

Also remove some duplicated code between public api and
system, by making a new make macro.

Paths before:
  frameworks/base/../../prebuilts/sdk/api/28
  frameworks/base/../../prebuilts/sdk/system-api/28

Paths after:
  prebuilts/sdk/28/public/api/android
  prebuilts/sdk/28/system/api/android

Test: make offline-sdk-referenceonly-diff
Test: make offline-system-sdk-referenceonly-diff
Bug: 77525052
Change-Id: I54989816ea09ad7a1e2f2355926678e6d2edb27c