SDK / exoplayer

Before modularisation of the code, it was possible to do an out of tree build
giving an -PbuildDir=someDir argument to gradle. With the modularisation, it's
broken as using -PbuildDir=someDir will force the same directory for each
projects, which breaks the gradle build system.

This commit adds a new externalBuildDir project property to allow out of tree
builds again. When set, it updates the buildDir property for each project to
point to ${externalBuildDir}/${project.name}. That way, the build artifacts are
written in the out of tree directory in a project specific folder.

To do an out of tree build, use

    gradle -PexternalBuildDir=someDir ...

It supports absolute and relative path. Relative path are interpreted against
the ExoPlayer root directory.