Installd: Check for dex2oatd before running it

Only select dex2oatd when it is actually available. Moves
a check to be a conditional.

Test: mmma frameworks/native/cmds/installd
Test: installd_dexopt_test
Change-Id: I4c5a8de68d0b4874632518733aeada22e1e6290c
1 file changed