Add buildDir to WritablePath implementations

First, the buildDir() method was renamed to getBuildDir() to avoid
clashing with the buildDir field.

Then, a buildDir was added to both `OutputPath` and `InstallPath` but
not to `PhonyPath` as it does not contain any path components. Instead
the `PhonyPath.getBuildDir()` was changed to simply return "".

Bug: 183650682
Test: m droid
Change-Id: I12e1854c829b980c5c01205753c62c00dc0a4774