Change the default for LOCAL_JAVA_LANGUAGE_VERSION from 1.7 to 1.8

Change the default for LOCAL_JAVA_LANGUAGE_VERSION from 1.7 to 1.8

Modify the compiler flags for Jack and javac.

This has the following effects:
1) Generally, some of the type inference rules changed.
2) javac: bytecode is generated with the v52 major version (not v51)
3) jack: Java 8 language features are supported.

The javac / dx toolchain does not support Java 8 language features.

Bug: 26753820
Change-Id: Icf619f601def30600a42a70815fc8cc754eafa8a