Add ability to skip creating intermediate R.java

Created by: raviagarwal7

of dependent libraries/aar for android_library target via skip_prebuilt_r_dot_java

• add skip_non_union_r_dot_java arg to android_library target.
• add resource_union_package arg to robolectric_test target.
• Only return RDotJavaFiles of union package when skip_non_union_r_dot_java is true.
• Only create union package’s R.java file and skip other packages when skip_non_union_r_dot_java is true.

This helps in improving the build times as only n R.java will be compiled now (1 for each library in the dependency graph), which earlier it used to be n * log(n).