Geneate AndroidManifest file if the apex_test module does not have one.

If the build file contains the apex_test module, but the apex_test
module does not contain the AndroindManifest file, then create the
AndroidManifest file.

In such case, the apexer tool is already generating an AndroidManifest
file. In order to handle the testOnly attribute for apex_test modules, a
--test_only flag is appended to the opt flags.

The apexer tool reads the opt flags and if the --test_only flag is
present then it generate the AndroidManifest file with testOnly
attribute.

Bug: 213310150
Test: m nothing && m test_com.android.sdkext
Test: manually checked the generation of AndroidManifest file in the
unsigned zip file mentioned in the description.
This command allows to read the attribute of the binary xml file -
aapt2 dump xmltree test_com.android.sdkext.apex.unsigned --file
AndroidManifest.xml

Verified the presence of testOnly flag in the output.

Change-Id: Ic47378428b2dba51d73e75d912546c2374f68d57