Revert "Removing flag com.android.wm.shell.enable_pip2 from trunk_staging." This reverts commit e2a47c79d498f427dd1ed989222f142d67a41d2e. Reason for revert: <Potential culprit for b/391286841 - verifying through ABTD before revert submission. This is part of the standard investigation process, and does not mean your CL will be reverted> Change-Id: Icd14920a8dbfdc2f256e9950101909d80410b44b