remove framework_base_subdirs

Metalava no longer requires it at least for droidstubs in this
directory. Even when local_sourcepath is not set, the -sourcepath option
is set to the module directory (frameworks/base in this case), and
metalava tries to find package.html files under the directory to know if
a package is hidden or not.

Exempt-From-Owner-Approval: cherry-pick from internal

Bug: 144264106
Test: m

Merged-In: Idfa2aa392ae1da9582e7691a8c06c986baad5088
(cherry picked from commit e53786e8)
Change-Id: Idfa2aa392ae1da9582e7691a8c06c986baad5088