Broken layout of `java_gapic_assembly_gradle_pkg` output

[aohren]

Up until recently, the java_gapic_assembly_gradle_pkg rule would product a .tar.gz packaging with the following directory hierarchy:

[Gradle build files + 3 top level directories (for a java_gapic_assembly_gradle_pkg with name = "googleads-java")]:

.
├── gapic-googleads-java
├── gradle
└── proto-googleads-java

Now, the above hierarchy is nested 1-level deep, and two new additional directories (gapic-googleads-java and proto-googleads-java) exist at root level (one directory up from the build files). The generates sources are also spread out now amongst these 4 different directories (where previously they were only in the 2 gapic-googleads-java and proto-googleads-java directories above:

.
├── gapic-googleads-java
│   └── src
├── googleads-java
│   ├── gapic-googleads-java
│   ├── gradle
│   └── proto-googleads-java
└── proto-googleads-java
    └── src

This has broken ability to consume these products downstream, and also makes the Gradle assembly unbuildable, since now there are source files outside of the gradle build directory hierarchy.

[vam-google]

This is googelapis-wide issue. Something is completely broken here. May be related to the update to protobuf 3.12.1

[vam-google]

Fixed