Update default Java level for idegen.

This sets it to 1.8 instead of 1.7 as Android now allows using
1.8 java language.

Bug: 38140365
Test: Ran idegen and now java language is set to 1.8 in Intellij
Change-Id: I1a1f0970357f42c3bc5ded9b3f17692812ea130a