Make javac always use Java 7 when running tests using javac and dx

This adds "-source 1.7 -target 1.7" to JAVAC when --build-with-javac-dx
is specified.

Also make sure that dx is not run when --jvm is passed.

Test: art/test/run-test --host --build-with-javac-dx 003-omnibus-opcodes
Test: art/test/run-test --host --build-with-javac-dx 005-annotations
Test: art/test/run-test --host --build-with-javac-dx 023-many-interfaces
Test: art/test/run-test --host --build-with-javac-dx 056-const-string-jumbo
Test: art/test/run-test --host --build-with-javac-dx 091-override-package-private-method
Test: art/test/run-test --host --build-with-javac-dx 111-unresolvable-exception
Test: art/test/run-test --host --build-with-javac-dx 113-multidex
Test: art/test/run-test --host --build-with-javac-dx 124-missing-classes
Test: art/test/run-test --host --build-with-javac-dx 126-miranda-multidex
Test: art/test/run-test --host --build-with-javac-dx 127-checker-secondarydex
Test: art/test/run-test --host --build-with-javac-dx 131-structural-change
Test: art/test/run-test --host --build-with-javac-dx 138-duplicate-classes-check2
Test: art/test/run-test --host --build-with-javac-dx 303-verification-stress
Test: art/test/run-test --host --build-with-javac-dx 551-checker-shifter-operand
Test: art/test/run-test --host --build-with-javac-dx 555-checker-regression-x86const

Test: art/test/run-test --host 003-omnibus-opcodes
Test: art/test/run-test --host 005-annotations
Test: art/test/run-test --host 023-many-interfaces
Test: art/test/run-test --host 056-const-string-jumbo
Test: art/test/run-test --host 091-override-package-private-method
Test: art/test/run-test --host 111-unresolvable-exception
Test: art/test/run-test --host 113-multidex
Test: art/test/run-test --host 124-missing-classes
Test: art/test/run-test --host 126-miranda-multidex
Test: art/test/run-test --host 127-checker-secondarydex
Test: art/test/run-test --host 131-structural-change
Test: art/test/run-test --host 138-duplicate-classes-check2
Test: art/test/run-test --host 303-verification-stress
Test: art/test/run-test --host 551-checker-shifter-operand
Test: art/test/run-test --host 555-checker-regression-x86const

Test: art/test/run-test --jvm 003-omnibus-opcodes
Test: art/test/run-test --jvm 005-annotations
Test: art/test/run-test --jvm 023-many-interfaces
Test: art/test/run-test --jvm 056-const-string-jumbo
Test: art/test/run-test --jvm 091-override-package-private-method
Test: art/test/run-test --jvm 111-unresolvable-exception
Test: art/test/run-test --jvm 113-multidex
Test: art/test/run-test --jvm 124-missing-classes
Test: art/test/run-test --jvm 126-miranda-multidex
Test: art/test/run-test --jvm 127-checker-secondarydex
Test: art/test/run-test --jvm 131-structural-change
Test: art/test/run-test --jvm 138-duplicate-classes-check2
Test: art/test/run-test --jvm 303-verification-stress
Test: art/test/run-test --jvm 551-checker-shifter-operand
Test: art/test/run-test --jvm 555-checker-regression-x86const

Change-Id: I60f8a973c0dd425988abe55223bdef426c772b97