Add @TestApi to ActivityManager.IMPORTANCE_CANT_SAVE_STATE_PRE_26.

Fixes: 112329019
Test: cts-tradefed run singleCommand cts-dev -m CtsAppTestCases -t \
      android.app.cts.ActivityManagerProcessStateTest
Change-Id: I1c6f1dd9503cde5b11e9d2c3c02841771b665660
2 files changed