Deprecate 'contributions' field from linkerconfig

Linkerconfig's contribution feature was introduced in Android T, but
it's not used actively (probably, never used?). Let's deprecate it. Note
that its runtime behavior is kept to support any potential uses.

Bug: 309716069
Test: linkerconfig_diff_test
Test: Add "contributions" field to an apex, build it.
      It should fail with an error message.
Change-Id: Icb91919142de14696219308674f1b3ac3e2d029b