Add testOnly attribute to AndroidManifest file of apex_test

If the build file contains the apex_test module, add the
testOnly attribute to the application element of the
corresponding AndroidManifest file and set its value to true.
If the testOnly attribute is already present and has value
false, then do nothing.

Tests added in manifest_fixer_test.py to check if the updated
AndroidManifest file has the testOnly attribute set to true or not.

Bug: 213310150
Test: atest --host manifest_fixer_test
Test: m nothing
Test: manually checked the AndroidManifest file generated
Change-Id: I36247dbe0261c342d451a4422c314fd8fe0c2369