Add only compiler-flag dependencies as implicit

Bug: http://b/72343691

Change https://android-review.googlesource.com/c/572758, in addition to
dependencies from the compiler flags, also marked all exported
dependencies as implicit.  This can cause lots of unnecessary
recompiles.  This change moves exported dependencies back as order-only
dependencies.

Test: 1. mma in art after changing profile_compilation_info.h triggers
         only a limited number of recompiles.
      2. verify that changes to PGO profile files trigger recompiles.

Change-Id: Icb0f4cd2b6da0add3b6e5206661e6aa7a577602f