Only enable isolated compilation if requested

Isolated compilation saves memory but takes much longer,
so enable it only if requested.

Test: adb install
Bug: 190277710
Merged-In: Ia1bffb37ff25f80f7b6173fd42e6bd6ffff68e45
Change-Id: Ia1bffb37ff25f80f7b6173fd42e6bd6ffff68e45
(cherry picked from commit c7ec2ca018f951b2bc65e99b27eab991da200097)
1 file changed