Grant Shell ability to clear cached app data.

It already has CLEAR_APP_USER_DATA to clear everything inside app
storage, and clearing cached data is a subset of that.

Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.StorageHostTest
Bug: 36731175
Change-Id: Iefc5be6c80e2562a95424fd6fe413bdb018201a9