Prebuilt APEXes are recoreded in apexkeys.txt

apexkeys.txt now correctly lists prebuilt APEXes with keys specified as
PRESIGNED.

This change also fixes a bug that non-installable APEXes are listed in
the file.

Bug: 131130235
Test: m out/soong/apexkeys.txt and check that
com.android.apex.cts.shim.apex is listed there with PRESIGNED keys.

Change-Id: Ib6d391a82864714743a1cc59cd655bea917b5073