Update maxTargetSdk for createAshmemBitmap

Bug: 150395371
Test: make

Follow-on to I359187a5c70b5e241c7f5879d50fde2a7449c818. Apps built
against R should have @UnsupportedAppUsage access to createAshmemBitmap,
since asShared does not exist yet. But apps built against S should use
the new API.

Note that the R branches have *no* maxTargetSdk set for this method.
This was added in S before it was possible to set R as maxTargetSdk.

Change-Id: If2313809084bc70613214b562847ae3a792df259